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Tsukasa knew that, from the moment he'd met Senku, he was in love. The way the younger boy talked about science and the way his sharp tongue could bring down almost anyone's ego was beautiful to the fighter. He wished he'd stayed on his side from the beginning. If he had, maybe then he could have been among Senku's most trusted. But he wasn't. Gen was.
The mentalist was always around Senku, his soft eyes almost always fixed on the scientist. Senku seemed to enjoy the man's presence, his crooked smile always on his face.
Tsukasa hated it. He wanted to be in Gen's place. He wanted to be the one that Senku always wanted around him. But his strength often meant he was needed to help carry heavy supplies around.
But that damn captain, Ryusui, was even more of an annoyance to him. The captain had been chosen by Senku and had gained his trust quickly. The blonde man was very affectionate with everyone, but especially Senku. Whenever he was near the other boy, his arm was always over the scientist’s shoulder or his hand would be resting on his arm, always close to him. Senku never pushed his hand off like he did others and would instead almost lean into it. The men seemed to almost read each other's minds with how often they were completely in sync.
He knew it wasn't their fault. He knew that it was his own fault for not siding with Senku earlier. But that didn't matter. What bothered him was the way Senku had started sitting with Ryusui and placing his hand on Gen's arm when they were near each other. Gen would often laugh more and lean into Senku's space, his eyes softening and sparkling when the scientist would lean slightly closer in return. Ryusui, being as bold as he was, had started flirting with Senku and would often cup his chin in one hand, gently pulling the scientist’s face closer as he quietly whispered praise and soft, teasing words to the man. Senku never pushed him away, causing Tsukasa's jealousy to worsen every time.
Every time Senku allowed Gen and Ryusui to flirt or to get close to him, Tsukasa could feel his heart ache. He felt even more jealous when he found out that the 5 Wise Generals formed while he was unconscious because that meant Gen and Ryusui got even more time with Senku and got to be even more trusted by him.
He wanted to be with Senku almost as much as he'd wanted to save Mirai, but he couldn't. He could only watch as Senku slowly ended up getting closer and closer to the other men. He'd often see Gen's arms wrapped around Senku's or Ryusui's arm wrapped around Senku's waist, holding him to his side while he flirted. Senku never pushed them away and would even lean into their touch, his hand sometimes making its way into their hair.
Tsukasa knew Senku wouldn't love him but he never gave up hope. He refused to. He held onto the hope he could eventually be the man Senku loved until that fateful day.
The Perseus crew was holding a party to celebrate Kohaku's birthday and most of the crew was drinking. Tsukasa, Senku, and Gen were among the only ones not drinking, instead keeping watch. When Chrome came to relieve Tsukasa of his watchmen duty, he went to find Senku but couldn't. Kohaku quietly told him Senku had gone down to his room, her teal eyes darting away from him as she said so.
Tsukasa quietly thanked her and headed to Senku's room, intending to talk to him but when he got near the scientist's room, he saw the door was slightly ajar. He pushed it open to see Senku pressed against him, his hands tangled in his hair as they kissed. The fighter felt his heart break as he watched Senku's hands gently run through the blond locks of the captain, Ryusui's hands gently clasped onto his waist.
Tsukasa slowly closed the door and went to his own room, his heart aching as he thought about the way Ryusui had held onto Senku and how Senku had tangled his fingers in the older man's hair. He had known Senku wouldn't love him back, but he hadn't been expecting to see the proof in front of his eyes like that. He knew he would have to move on but, when he saw Senku the next morning, his crimson eyes sparkling softly as Ryusui sat next to him, he subconsciously knew he'd never move on from the scientist. He would pine away for the man forever.