Author's Note: First and foremost, I don't own Fire Emblem or any of its characters. I make no money off of this. This is just written for fun.
Lysithea turned out to be my favorite character in Three Houses, and with it being her birthday, I decided to write this small one shot for her. This is a small AU (Alternate Universe) where it turns out Crests can't be removed from the body. As such, Lysithea feels like she's doomed to live the short amount of time left, but it turns out having a Goddess with power over time living within your consciousness has some solutions to otherwise impossible problems.
This story is a simple retelling of the Byleth / Lysithea S-support with the above premise. Nothing fancy, but I hope you enjoy. The route is left ambiguous on purpose, so feel free to use whichever ending you want.
Byleth walked up the stairs to the Goddess Tower, expecting to find Lysithea. He had asked her to meet him at the tower to ask her two important questions, ones that will shape the future of their lives. After the war ended over a month ago, he decided it was time to finally meet with his most precious student and give her something that he's been meaning to for a long time now. Despite his best efforts, he and the rest of the students and faculty had yet to find a means of removing one's crests safely. He ended up searching for a solution within and found an unexpected answer from someone he thought he'd never hear from again.
Earlier this day
"So it's the youngest one that holds your heart, I see." Sothis's voice chuckled in his head.
"Yes. But without a means to remove her crests, she only has a little bit of time left. 5 years at the absolute most." Byleth replied, with a slight frown.
"Ah, but have you forgotten who I am and what I'm capable of already? Of course you did. I am a Progenitor Goddess, with power over time itself. Rewinding time isn't the only thing I, and you by extension, am capable of."
"What do you mean?"
"Ever notice that when you're using my Divine Pulse, time stops completely for everyone but you? You're still able to notice the world, even though you aren't able to interact with it without resuming time. It is how you were able to make such complex decisions, even without having to rewind time."
"I know this. What's your point?" Byleth was still uncertain where this was going, but he was slowly piecing things together now.
"The reason you're able to take the time to make plans and think things through is because my power is resonating through the entire world. But what happens if we were to focus that time stopping power onto one single point and have that part be stopped in time? Or, more accurately, one person's life force?" A small smirk appeared on Sothis's face.
"But it's not that simple, is it."
"Yes, you catch on quick. There will be two, shall we say, side effects to using my power in this way. First off, with my full energy going into this special Pulse, you will be unable to use the power yourself without releasing hers. Doing so will no doubt cause all of the stopped time to accelerate forward, which will likely kill her if you do this after her natural alloted time."
"That's fine. I've lived as a mercentary without this power before, and I can do so again. What's the other?"
"Secondly, this will bring her body's internal clock nearly to a complete stop. She will no longer age as normal but will also be unable to experience things that require a body to change, such as bear children. Her life will be as long as yours or until you release the Pulse that we will place upon her. That's the best I can do for you." Sothis smile falls at this.
"I will speak with Lysithea about this. Thank you once again, Sothis."
With that, Byleth set out to find his favorite student to give her this choice.
"Professor!" Lysithea's eyes lit up as Byleth approached. Ever since he asked for her to meet him here at the Goddess Tower this evening, Lysithea has been nervous. Her teacher seemed to be in a much better mood today, even if she was the only one that noticed the difference. Whatever he wanted to say will no doubt be good news, she believes.
"Good evening Lysithea. Thank you for meeting me here." Byleth took a few seconds to compose himself.
"It is no problem. But why have you asked me to come here?"
"I have two things to ask of you. But before that, how have you been?"
"I'm okay, all things considered. Even with Linhardt's and Hanneman's best efforts, it seems like I'll have to carry this burden for the small amount of time I have left. I will decline my family name and allow my parents and those that have served us to live in peace for the short time I have left. But enough about me, Professor. What did you call me here for?"
"Alright. First, I'd like to give you this." Byleth handed her his mother's ring. It was a simple, silver band with amethyst gems in the shape of flower petals around the center.
"This is..." Lysithea's eyes widened significantly.
"This is the last memento of my mother. My father gave this to me, in the hopes that I gave it to the person that I love the most one day. I now give this to you, Lysithea. I love you, and I wish to share the rest of our time together."
"I... I can't." Lysithea slowly breaks into tears, whether they're tears of happiness or sadness is difficult to say at present. "I only have a few years left at most. I want to be with you, but I don't want to put your through the pain of losing me so quickly." With that, Byleth pulls his hand back. But he keeps his palm up, ring in hand, for her to take whenever she wants to. But when she looks at his face, she notices that he's smiling rather than upset as she expected.
"Maybe we have more time than you think. Let me tell you a secret that I've kept ever since I became a teacher nearly six years ago."
With that, Byleth explained to her everything about himself. His stillborn birth, being saved by Rhea who implanted the Crest of Flames onto his heart. The dreams of his childhood, especially those of a girl on a throne. The young girl's awakening and saving of his life when he was about to take an ax for Edelgard. Her power to stop and reverse time in short bursts. Her merging with Byleth at the Sealed Forest to break Solon's spell. The five year gap where he recovered before she told him to return to his students. And finally, the last revelation that the goddess gave earlier this morning.
"So you've had the Goddess Sothis's power all along, and you've never told anyone? And she can really do this for me?"
"Yes. But as I said, there will be some consequences in doing this. Do you want to live with me, as you are now, for as long as I'm alive?
"Of course I do! I'd be a fool to turn this down. But what about you? By keeping me alive, you'll be without your power for the rest of your life, and I can't even help you continue your family name. Why give all of that up for me?"
"I have told you. I've come to love you above all others, Lysithea. I'm more than willing to give up something like this to be with you for the rest of our days." With that, he reached out his hand that held the ring towards Lysithea once again. "Take this as my promise to you."
As Lysithea reached out and grabbed his hand, a powerful pull of magic flowed into her, along with a young girl's voice saying "Your will and mine are now as one. Pulse of the Divine, lock this body's life force in time!" Her world flashed in a mix of purple and blue for a few seconds before everything returned to normal once again.
"It is done." the strange voice, Sothis as Lysithea learned, says. "As long as you don't use any more Divine Pulses, her body's life will remain as it is now for all time, or at least until you die."
"Thank you, Sothis. This means everything to me." Byleth replied.
"Yes. Thank you, Goddess Sothis. I promise to never leave Professor Byleth's side as long as we live."
With that, Byleth pulls Lysithea into an embrace as she clings desperately to him, never wanting to let go ever again. The sun begins to set behind Lysithea, yet neither of them make an effort to move. They now have each other for the rest of their days, and that is more than Lysithea can ever ask for.
"From now on, we will chase that future. Together, my precious Lysithea."
"Yes. I quite like the sound of that. Together! We will be together forever. Living long, full lives! Count on it, my love, my professor, my life, my Byleth."