The first time Vanitas did something for Ventus, he didn't even have a name of his own yet.
He was Darkness, he was part of a "we", and he was fascinated by the boy in front of him. At first, he thought it was the same interest his kind always had for the denizens of light. Then that it was fate, as Ven was destined to host him someday.
Still, when he had given Ven what he wanted more than anything—a position, power, importance—he couldn't deny there was a part of him that took some kind of satisfaction in the work. Something that ran deeper than just knowing he had gotten a human exactly where he wanted them.
It must just be something novel about their linked destinies. He was Darkness, one of many, and he didn't have such simple, human desires. Nonetheless, he promised—to himself, if no one else—to chase that sense of satisfaction as best he could.
And if it meant he could stick around and watch Ventus giddy at the prospect of making friends, well, that was just part of the plan.
The second time Vanitas did something for Ventus, he told himself it was out of self-interest. Xehanort was about to discard him completely for his inability to wield the darkness Vanitas was supposed to provide him with; but instead, the Keyblade Master sifted Ventus's heart in two clean halves, and Vanitas was brought to light.
He was given an opportunity—to witness a Keyblade Master who served the darkness. To learn, himself, to wield a Keyblade of his own. To see darkness triumph in the Keyblade War, perhaps. Any one of his kind would have seized this unbelievable chance.
And if it meant Ventus was safely preserved, even as a pawn in Xehanort's plan, well, that was just lucky for him.
Being given human form came with strange phenomena. These newfound emotions that Vanitas had never experienced before—and which decided to take a physical shape in the Unversed. Xehanort said the χ-blade could help Vanitas find the answers he sought; Vanitas doubted that was the case, but it was worth trying.
But through the haze of pain, and misery, and sorrow, and despair, there was the presence of Ven at the core of his heart. The Unversed might be a way to toughen him up, and if Ven grew stronger, they could create the χ-blade together.
So the third thing Vanitas did for Ventus was just that. Draw him out of his golden cage, force him to grow. It was what Xehanort had done for him, and that sometimes sounded like caring, even if it mostly felt like cruelty. At times, Vanitas even wondered if what that meant about what we did for Ventus.
And if it meant he would meet Ventus again, well, that was just means to an end.
The fourth time Vanitas did something for Ventus, he woke him up from a slumber that could have been eternal.
He couldn't claim all the credit, of course. The Guardians of Light played their part. Sora, the bearer of Ven's heart and holder of the Power of Waking. Aqua, as the motivation for Ventus to come back.
Still, he couldn't suppress a hint of resentment that there was no gratitude saved for him—only a look of anger on Ventus's face that hurt worse than any Keyblade. As if he would really have slain one of the Guardians right there and then. As if he wouldn't have done everything to make sure that this boy would wake up, same as them—perhaps even more so. He just worked in his own ways.
And if it meant Ventus was awake again, Vanitas would take the anger. It was one of the emotions he knew best, after all.
One thing Vanitas didn't expect was for Ventus to do something for him, after all this time.
So when he was brought back—torn from primordial darkness and restored to his body and his identity—he had no idea how to react to Ventus's smile, or to the hand he was holding out to him.
But there was that satisfaction again, as it came every time Ventus smiled this brightly, and Vanitas knew that that smile was his somehow. And he had made a promise to himself. So he took that hand.
And if it made Ventus's smile just a little warmer, well, that explained the heat on Vanitas's cheeks.
Vanitas didn't expect that the next time he would be doing something for Ventus, it would come this naturally.
It had taken some time figure out the puzzle of Ventus's kindness, of the things Vanitas felt but couldn't name, of that thing he chased that he called satisfaction but probably had another name. By the time he knew what it was, he was afraid to speak its name—a fear that was different from the turmoil of darkness he was used to.
But he was brave enough to put his lips on Ventus's lips, at least. Brave enough to meet Ventus's gaze after they came apart, finding it filled with sharp, tangy surprise, and something sweet whose nature Vanitas didn't dare to assume.
"There is a promise I made to myself, a long time ago," he said. "I wasn't sure what it meant at the time, but I knew what I wanted." He paused. "I promise to do anything for you. Anything you want. Anything you need." He caught the pained look in Ventus's eyes, and winced. "And I promise I'll make sure you actually want me to do it, in the future." He cleared his throat. "Including…this just now."
Ventus's face broke into one of his easy smiles that Vanitas would never grow tired of. "At the very least, this just now is fine, moving forward. Just so you know."
It wasn't until relief washed over him that Vanitas realized how panicked he'd been. "Good. I was hoping for that." Vanitas was pretty sure he would have collapsed if it weren't for Ventus's hands on his shoulders. "There's so much more I want to tell you, but—I don't know how."
"That's all right," Ventus simply said. "We've got time. And I can help you learn."
And if Ventus kissed him back just then, well, it was everything Vanitas wanted and more.