Sora stared at the river in front of him. Its waters were dark and murky, and the last thing he wanted to do was jump in. A river whose name sounded the same as a pile of sticks shouldn't be so creepy, right? But it was, and there was only one reason he was here. Herc had told him bathing in this river would make him almost completely invulnerable… if he survived the plunge.

After causing his friends grief for over a year, invulnerability sounded pretty good. He couldn't abandon them again. He couldn't. Especially not Kairi. The tear sliding down her cheek as he faded away was an image that would forever be seared into his heart and burned into his brain. It drove him to act, to spare her from further pain down the road.

Still, a shiver went down his spine as he watched the churning waters. This wasn't the first time he'd been to the Underworld, but before he'd had people with him. Now he was all alone because this was the kind of thing that had to be done alone.

As he took his shoes and socks off, Herc's words from before echoed through his mind. "Careful, Sora. The river Styx makes you forget. It makes you lose yourself. Your heart has to be stronger than it or you'll be washed away."

"My heart is strong because of them," Sora murmured as he removed his jacket and set it next to his shoes and socks. He was even colder without it here in this dark, damp cavern, but he hadn't grown up on a world full of islands for nothing. Anything that might make it harder for him to swim had to go. He kept his thoughts on his friends, and that distracted him from his discomfort.

"You'll need an anchor, a lifeline," Herc had said. "A spot on your body that connects you to the mortal realm. It'll be your only weakness, but it'll keep you human."

Sora took off his tank top and thought carefully. There was that statue he'd knocked over in Thebes by hitting its heel. Of some famous guy named Achilles was it? Yeah, that sounded right. According to Meg, Achilles had done the same thing Sora was about to do—he'd bathed in the river Styx. His only weakness was his heel, and someone had shot an arrow at it during battle, so he'd died.

"Not gonna make that my weakness," Sora said, chuckling nervously. "Too obvious."

Any spot on his limbs was out of the question, as his arms and legs were constantly exposed to danger as he fought. That left somewhere on his head, which was also a bad idea, given his enemies' tendency to target it, or his torso.

His pants joined his pile of clothes, leaving him in nothing but his swim trunks. He had this stupid idea, this ridiculous, dumb idea about where his weak spot should be. It was his weakest spot, after all, and maybe his enemies would hear about his invulnerability and think he couldn't possibly be stupid enough to let this be his one weakness. They'd target him everywhere but the most obvious place of all.

He took a step closer to the river and stared into its murky depths. What would it feel like to jump in? Would it hurt? Would it feel weird? The longer he stared, the more uneasy he grew. Well, better act now before losing his nerve. Otherwise he might not go through with this.

Taking a deep breath, he steeled himself and plunged into the water. Instantly he regretted this entire plan. Instead of feeling wet or cold, his body felt like it was on fire, burning from the inside out with nothing to stop it.

"Sora! The river will turn you to ashes if you're not careful!" came Herc's panicked voice, but it seemed so far away.

No kidding! Sora wanted to yell, but he couldn't speak. He couldn't breathe.

Naminé's scared face appeared in his mind. "Sora, hold on to your memories!"

I'm trying! Sora wanted to say, but even as he responded, he felt them start to slip away, like grains of sand pouring out of his hands.

"You make a good Other."

Roxas—

Wait, who was Roxas? Sora panicked and struggled and thrashed, his lungs burning. He couldn't remember. He didn't know who… who…

"Giving up already? C'mon, -, I thought you were stronger than that."

His mouth opened in a wordless cry. He couldn't remember who was speaking to him. He couldn't even remember his own name. It was all gone. He was falling, falling into darkness. It engulfed him and consumed him, and maybe it would be best to just let it swallow him whole. Then he could fall into an endless sleep. Rest for all eternity.

Rest, that was what he needed. He was so tired… so tired of fighting… Sleep sounded good… He was floating now…

"Sora!"

His soul flew back into his body and his heart thundered in his chest. That voice. He knew that voice. His eyes flew open, and she was leaning over him, a big smile on her face. Her red hair was a little tousled from the ocean breeze, and her violet eyes were shining.

"Kairi!"

"Sora, you lazy bum," she greeted in return. Her melodic voice was full of affection for him, and warmth pulsed through his heart and body with every note. The hot island sun beat down on them both, and he could hear the waves rhythmically pounding against the shore.

"Still snoozing after all this time." She giggled into her hand, his favorite Kairi giggle. "What did I tell you about falling asleep everywhere?"

He scratched his cheek. "Sorry."

"It's okay," she said softly. "I'll always be there to wake you up." She offered her hand, a tender smile on her face. "Remember who your lifeline is."

His eyes went wide and his lips parted. "It's you, of course it's you. You're my strength, my anchor to my humanity, my light in the darkness."

The tiny spot on his chest where the Keyblade of Heart had first pierced his body flashed, and for a moment he saw the cord that connected him to Kairi. It was thin but sturdy, red as blood, and it came from his heart and connected him to her heart. He grabbed her hand, and she pulled him up, up, up out of the darkness. His memories came back to him in a flood, and he staggered out of the river. He gasped for air and rested his hands on his knees, glad to be alive.

Kairi wasn't there, but the burning was gone. The pain was gone. His ordeal was over, and he'd come out stronger for it. He touched the tiny spot on his chest, his one weakness. His greatest strength.

"Kairi, thank you," he murmured, his heart beating out a steady rhythm. The part of him responsible for keeping him alive, vulnerable despite all its power, was in Kairi's hands now.

He knew he could trust her to keep it safe.

Kairi's training with Aqua had paid off; she finally felt like she was helping Sora instead of acting as a burden or a liability.

Still, this battle was tough. She wiped her sweaty forehead as she and Sora regrouped behind a pile of stones. They were fighting another one of those masked people. The Foretellers, Ven called them. They had darkness trapped inside their hearts, and fighting them was the only way to defeat that darkness and free them.

This Foreteller wore yellow and gray robes with a leopard mask. They'd tracked him down to the ruins of an ancient palace that the sprawling jungle of this world was trying to reclaim. Kairi wondered about the people who used to live here and when the palace had been abandoned. They'd seen nothing but animals until now, so she wasn't even sure there were still people here except this Foreteller.

"Where's Ava?" the Foreteller cried from his perch on a crumbling wall. "Where is she?"

His Keyblade was gripped tightly in his hand. Kairi thought it looked like a series of lightning bolts strung together, and the keychain with its creepy blue eye hadn't escaped her notice either. Nor had the darkness seeping off of him.

Sora stepped out from their hiding place. "Ava, is that one of your friends? What's your name?"

He panted for breath, his body coiled up like a spring and his hand shaking a little. Kairi knew he was bracing himself for the battle to continue even though he was trying diplomacy first.

"Gula," the stranger snapped, "but what does that matter to you!"

Before Sora could react, Gula charged. Normally, Kairi wouldn't be worried. Sora was invulnerable now, and he was an incredibly skilled fighter. When he was with her, she had no doubt that they would win, that he would keep her safe, that they would succeed.

Until now. Panic struck her gut like the lightning bolts on Gula's Keyblade. Her heart thundered in her ears as time slowed down around her. Sora was in grave, mortal danger. She wasn't sure how, she wasn't sure why, but she knew he was. Taking a deep breath, she latched onto her connection to him, a small red cord that bound them together. She was now his eyes, his hands, his heart, his body. She controlled his arm and held out his Keyblade, guarding his heart just in the nick of time, and Gula's Keyblade clashed into his with a loud clang.

"You leave him alone!" she shouted, her heart fully in control of Sora's body now. "I won't let you hurt him!"

All her grief over losing Sora had transformed into blind rage and an uncontrollable urge to protect him at all costs. She wouldn't let him die. She wouldn't. Gula would have to destroy her heart first, destroy her, before he could hurt Sora.

To her shock, Gula's mouth dropped open, and his Keyblade slipped from his grasp. He looked as if he'd seen a ghost.

"You sound… Just like her."

Kairi tilted her head. "Huh?" By 'her,' did he mean—

"Kairi, I wanna talk to him too," she sensed Sora telling her. But by the time she'd relinquished her hold and her heart had returned to her body, Gula had already disappeared into a swirl of dark smoke. She was still sitting behind the pile of rocks, as her body had waited patiently for her heart to return.

"That was dangerous," Sora said as he rounded the corner. His eyes were wide and his face was ashen, and his hand shook a little as he helped her to her feet.

"I know, I know, I'm not supposed to do that when my body could be left vulnerable." She brushed off her skirt. "But you were in danger. How? I thought you were supposed to be invincible."

"I am. Mostly. But I have one weakness. One small weakness that's the price to pay for being almost immortal. I had to choose it when I bathed in the river Styx."

She searched his face, then noticed he was resting his hand over his heart.

"Your heart? Your heart is your weakness?"

He chuckled nervously and scratched his cheek. "Well, more like one tiny spot over it—"

"You dummy!" she cried, and he grimaced. She couldn't believe he would choose a spot over his heart as the only place he could get killed. "What on earth were you thinking? You picked the most vulnerable organ in your body to be the one place our enemies can kill you? Why?!"

"I needed an anchor point," he said softly. "A lifeline."

Her lips parted. This lifeline he spoke of, was it that red cord she'd seen? She thought that was because they'd shared the paopu fruit, but maybe there was more to it now.

"My greatest weakness," he continued, "but also my greatest strength." He smiled tenderly and cupped her cheek. "Something that'll keep me human, that'll connect me to the Realm of Light. Or should I say, someone. It's you, Kairi. You're my lifeline."

A soft cry escaped her, and a lump built in her throat.

"Me? But I—"

She took a deep, ragged breath. Sora just smiled and caressed her face. How could he be so calm in light of all of this?

"Your heart is your weakness because of me?" she choked out.

"Well, I guess I could've picked any part of my body, but my heart made the most sense. It just felt right."

"How come?" she pleaded, barely above a whisper.

His gaze was tender and his expression vulnerable as he looked her straight in the eye. "I pierced my heart for you before," he said softly, "and I would do it again."

She teared up and threw her arms around him. Hugging him always felt amazing, and he wrapped his arms around her and held her tight.

"Oh, Sora," she murmured into his shoulder. "I'll make sure you never have to hurt yourself for my sake again."

He was silent for a few moments before he responded. "I know you will," he finally said, his voice low, an echo of the words she'd told him as fate had torn them apart. Had it really been nearly three years ago? It felt like an entire lifetime ago.

"Someone has to," she said, squeezing him as tightly as she dared. "Only you would go to all the trouble of making yourself invulnerable while still keeping your biggest vulnerability."

He chuckled. "Classic me, isn't it? Doing something stupid and reckless to show you how I feel about you." He kissed the top of her head and murmured, "But I mean it, Kairi. My heart's yours. I just thought I'd make it official."

She pulled away from him a little and gazed up into his eyes. "And I'll always protect what's mine." She smiled, a bittersweet smile, and found his hand. "Come on, let's go somewhere safer and regroup for now. Gula's long gone. We'll have to track him to a different world."

Besides, there was something she wanted to do for Sora, something she'd learned during her training with Aqua that she wanted to put into practice.

A way to fulfill her vow that he needed now more than ever.

They found a cave to shelter in for the night, and it was a good thing too, because it was pouring down rain in the jungle now. Sora got a fire started and set his jacket next to it to dry, and Kairi likewise had her dress drying too. She was in her tank top and shorts as she snacked on some nuts and berries they'd found, looking relaxed and at peace.

She was beautiful, sitting in the light of the fire like this. He couldn't stop staring at her.

"Um, Sora," she asked presently, her cheeks flushing as she studied her toes. "There's something I'd like to do for you."

Heat creeped up his cheeks too. "Oh yeah? What is it?"

"Take off your tank top, please," she said shyly.

He gulped. She'd never asked him to do that before. There were certain unspoken barriers they hadn't crossed yet, and undressing in front of her would definitely be toeing the line. She'd seen him without a shirt plenty of times of course, but that was at the beach. This was different. This was undressing at her request.

"Don't worry, it's not what you think," she said, then giggled into her hand. He couldn't fight the sinking feeling in his stomach, and that made her giggle even harder. "What, were you hoping it was?"

"Don't tease me," he said with a pout, his face hot. He leaned against the cavern wall, arms crossed as he tried in vain to stay irritated with her.

She cuddled up next to him, and he just had to raise an eyebrow.

"This 'isn't what I think it is,' but you tell me to start taking my clothes off, and now you're cozying up to me?"

She grinned and tugged at the fabric of his tank top. "Want me to take it off instead?"

Great, now he was really blushing. Kairi laughed as he fumbled with the fabric and got the tank top off. He wadded it up and tossed it into the corner, and she just shook her head and retrieved it. She set it out by the fire to dry, then smiled at him. Her gaze lingered for a few seconds, flickered up and down, taking him in.

Then she returned to his side, and when he held his arms out, she sat on his lap and they embraced. He had to admit that this felt really, really good.

"There, that wasn't so bad, was it?" she said softly, playing with his hair.

He shook his head. "Not at all."

She touched his chest as he gazed at her. Gentle, searching touches, though what she was looking for, Sora wasn't sure until she landed on a particular spot. His breath caught, and she stroked it tenderly.

"There it is. Your weak spot."

"Um, yeah."

Her lips pulled into a tight line. "Your heart's racing, are you okay?"

"I'm fine, I promise." She was the one making his heart race, surely she realized that.

Her eyes fluttered shut and she murmured some kind of incantation. A soft light glowed from her chest, pulsing with her heartbeat. The red cord that bound them together appeared, and he wondered what was going on. This must be some kind of magic Aqua had taught her.

"Kairi?" he asked.

She smiled and opened her eyes. "It's okay. This is a protection spell. Very powerful, but it's my first time using it, so I want to make sure I get it right."

"Protection?"

"For any weaknesses," she explained. "It's related to my abilities as a Princess of Heart I think. So long as I'm safe, your weak spot will be too. If I can cast this correctly, anyway."

She bit her lip and concentrated hard, and he cupped her cheek. "Kairi…"

She traced a pattern into his skin with her finger, then leaned down and kissed his chest. His breath caught again because her lips felt amazing against his skin. Her light flooded into him from the kiss and bound itself to the weak spot, forming a barrier and shield around it.

She pulled away, eyes still closed, a blissful smile on her face. "Sora," she said softly, and a jolt went through him as his name completed the ritual. He touched his chest, and his weak spot no longer felt like anything beyond a gentle caress would hurt him. When he removed his fingers, there was a small heart engraved on his skin as well as a series of tiny, tender letters inside it. He blushed and touched the letters, smiling as he read them.

She opened her eyes. "The Maiden's Kiss," she explained. "It only works on one person at a time, so I can only cast it on you. It'll protect you from all harm so long as I'm safe."

He lifted his hand, and she saw whose name was engraved on his chest. Her eyes went wide, and her face flushed. "Sorry, I didn't know the spell would brand you with my name!" She bit her lip and ducked her head. "I can try to remove that—"

He put his hand under her chin and gently tilted it up so she met his gaze. "Don't."

She stared at him for a few seconds, her cheeks burning. "If it makes you feel any better," she finally said, "your name is probably written over my heart now too." She clutched her tank top. "I know something is written there, I can feel it—"

"Even if it isn't," he said, loosening her grasp, "I wouldn't mind." He brought her hand to his lips and kissed it. "I told you, my heart's yours."

She smiled and brushed a strand of hair out of his face. "And mine's yours."

His lips found hers, and he kissed her deeply. They were still trying to be discreet about their relationship around their friends, but in the privacy of this cave, they were free to indulge in the happiness they had been denied for so long.

Whatever lay ahead of them, they would always look out for each other.

A/N: Happy, Happy Birthday to my dear friend Alja ! Your friendship and support means a lot to me ❤️Thank you so much, for everything—writing feedback, emotional support, fascinating discussions—it all means so much to me ❤️I hope you enjoyed this story!

Also, a big thank you to Liv for looking over the story!

And to everyone else, as always, thank you for reading! Alja was recently re-reading the Percy Jackson books and shared some Percabeth-inspired SoKai headcanons with me that we then spun into a couple of fic ideas, this being one of the results ❤️I first read the Percy Jackson books years ago, and they hold a lot of special memories for me, so it was nice to go on a trip down memory lane. I hope you liked the story!