The rest of this fic is travelling around the new chapters of the manga, so back when Isshin is the Captain of the Tenth Division. Seeing as we're still in that flashback and haven't seen much of Soul Society during that time just yet, I'll make up a bit as I go along until more is revealed.
Having done the maths for this, I've come to the conclusion that Kaien has already died, but don't ask me how I came to this conclusion because I'm not too sure myself. So if I'm contradicted by the manga, I'll work with what I'm given.
Countless centuries had passed since Ukitake and Kyōraku had graduated from the Academy and became Captains of their own respective Divisions. During those centuries, Jyuushiro had chased after his best friend, so in love with him it killed him inside to be avoided like he was the plague.
They both were still best friends, but Shunsui was so distant from Jyuushiro, the younger male sometimes wondered why he was trying so hard. The brunet just wouldn't allow them to touch each other, and if they so much as brushed shoulders by accident walking side-by-side, Kyōraku would jump back and stare at Jyuushiro in disgust as if the frail Captain had attempted to molest him.
Jyuushiro hurt. There was no simpler way to explain it. His heart ached every day for what he couldn't have, to have his best friend back, to close the distance he had created in their relationship. But most of all, he wanted the pain to stop. He always felt so dirty and unloved, like he was filthy for the things he wanted – for the things he had done with Shunsui, but deep down, he knew they were what he needed.
Many people looked at Jyuushiro in the way he looked at Shunsui, but he didn't want anyone else; he wanted his best friend. He just couldn't see himself with anyone else. But he knew he needed to stop this or he was going to go insane. He needed to stop pining after someone who would never love him back and find someone who will. He knew that – so why did it hurt so much to know that he needed to move on?
With a sigh, Jyuushiro elegantly signed his name on the bottom of a report his Sixth Seat had written, sometimes wishing that he could just die and get this all over and done with.
"Excuse me, Captain?"
Jyuushiro looked up at his guest's voice. He smiled softly, seeing a familiar head of white hair pop into his office. "Hitsugaya-kun. What can I do for you?"
The Third-Seat of the Tenth Division shifted uncomfortably before he spoke again, something that Jyuushiro recognised as very out-of-character; despite how young this boy was, freshly out of the Academy, he was calm and level-headed. Had something happened to upset him or make him uncomfortable?
"I..." The boy's shaggy white hair shook from side-to-side as Hitsugaya Toshiro sighed heavily. "...I have a question..."
Jyuushiro smiled in an encouraging manner. He opened the desk drawer and fumbled around for a few seconds before his hand resurfaced with lolly wrappers. He thrust them at the boy, his eyes bright and shining for once; he really liked Toshiro; the boy's name sounded like his and they both had white hair and green eyes. Toshiro was like the child he had always wanted but couldn't have.
"I don't like sweets..." Toshiro mumbled, uncomfortable as his question weighed heavily on his mind.
"Please." Jyuushiro's smile only widened, an excited look in his eyes.
Toshiro sighed and took the candy. He had no intention of eating them, but he politely slipped them into his pocket let he hurt his elder's feelings.
Jyuushiro smiled and sat back, happy to listen to the young child. "I'm ready if you are, Shiro-chan."
Toshiro bristled visibly but did not comment on his nickname. He instead took a deep breath, just wanting to get everything off his chest. "...How do I know if I am... homosexual?"
Jyuushiro was surprised by this question; out of everything he had been expecting... "...What brought this on, Shiro-chan?"
"Well, everyone says that you're a homosexual..." Toshiro started to explain, a childish innocence surrounding him. Jyuushiro couldn't bring himself to be mad or uncomfortable with the questions; Hitsugaya was very young and may not have understood these things properly. He would need patience in order to learn. "...When I asked what a homosexual was, they said it was... a man who likes other men... I know that homosexuals are not liked very well, and I was worried..."
Jyuushiro's eyes softened at the scared tone in Toshiro's eyes. Shinigami or not, this was still a child who was still coming to terms with such important facts of life. He understood the feeling of worrying about your sexuality; he had wanted to die for so long because he couldn't help who he was attracted to. He couldn't let the same thing happen to Toshiro.
Getting up from his chair slowly, Jyuushiro made his way over to the now-shaking boy. He wrapped his arms around the child, whispering into his ear. "What are you worried about?"
"...If I am... homosexual or not..." Toshiro whispered back. "...I... I don't like women... I never have... But I find myself looking at you more than I should... And if you're really a homosexual, you can help me. ...Right...?"
Jyuushiro nodded. "That's right. Please, do not worry about me pushing you away because of your sexual orientation; I would never do that to you. Feel free to tell me anything you feel comfortable with."
"...I don't understand... I've heard of cruel things happening because of who men like... What if it happens to me...? What if I really am... one of them...?"
"Don't say it like it's a bad thing," Jyuushiro soothed. "You cannot help what gender you are attracted to. I promise you that. There's nothing at all wrong with liking another boy."
"...I'm scared..." Toshiro confided. "...So scared..."
"There's nothing to be scared of. I promise. There's nothing wrong with you at all. You're normal."
Toshiro buried his face into his elder's chest, taking in the soothing scent of tea that always clung to Jyuushiro's clothes. He sighed softly, feeling at ease in these skinny arms.
"...Will you be disgusted with me for liking you...?" Toshiro couldn't help but ask.
Jyuushiro couldn't help but feel his heart shatter at these words, remembering as clear as crystal the first time he had revealed his feelings to Kyōraku; the feelings of being unwanted and disgusting, as if he were something that was unnatural... He couldn't let Toshiro feel those same feelings.
"Of course not, Shiro-chan. ...When I first discovered my feelings... I told my best friend... It was him who I liked... But... ever since then... we've been distant... I feel like I appal him for who I am... He doesn't treat me the same... I love him so much... But he'll never return my feelings... Two-thousand years later..."
"Captain Kyōraku... right...?" Toshiro whispered. He frowned when Jyuushiro nodded. His tiny hand reached up, wiping away the tear that threatened to fall from Ukitake's eye. "I thought so. I've seen the way you look at him."
Jyuushiro nodded. "...I know it's silly... I know I need to move on... But it's hard..."
"...Can you love me, Captain Ukitake?" Toshiro asked, his voice wavering slightly. He looked away when the older man fixed his gaze on him. "...I-I know you love... him, but... W-we can try together..."
Jyuushiro was silent for a few seconds, thinking over these words. He didn't want to feel as if he was merely using Toshiro for his own pleasure, but... He could at least try, as Toshiro had put it. If it didn't work out, he could end it. If it worked out... well, he could move on like he had been trying to.
Toshiro smiled, a bright, childish smile full of innocence. He leant up on his toes and placed his lips against Jyuushiro's cheek, kissing gently.
Jyuushiro blushed but didn't pull away. He smiled down at the boy, running his hand through the messy hair. As Toshiro moved back into his embrace, he couldn't help but hope he was doing the right thing; he didn't want to take advantage of Toshiro and his innocence.