There weren't many things in his life that Red regretted.
For example, he didn't regret that he'd nearly destroyed his relationship with his best friend all over a petty rivalry. He didn't regret battling Green that day on the S.S. Anne, or that day in Lavender Tower. He didn't regret defeating Green that day at the Indigo Plateau. He didn't regret that he'd never fulfilled the Professor's wishes and completed the Pokedex. He didn't regret that he'd stopped caring about much of anything. He didn't regret focusing on strength, constantly testing himself and his team against stronger and stronger Pokemon. He didn't regret wanting to be the best.
He did regret becoming the best.
At one time, Red had sought out other trainers. However, they began to pose less and less of a threat. The adrenaline that battle had once supplied him with decreased alongside his chances of losing. With time, Red began to avoid the other trainers. No one was ever good enough to compete.
And so Red turned his attention to the so-called legendary Pokemon. The Winged Mirages appeared to him first, but in the end they posed no threat. Even Mewtwo, the genetically enhanced replica of Mew, origin of all Pokemon, could not reignite Red's interest in battle. Red continued to search, however, desperate to find some challenge.
He did regret ever meeting that Pokemon.
Red wasn't even sure it was a Pokemon. He remembered that time and space themselves seemed to warp around the entity, but no matter how he tried, he could not recall its appearance. It had shrieked, like some large bird, and the sound had brought him to his knees. Though his memory of the event felt distant and foggy, he could remember that the pain had been unbearable. Even after the monster's shrieking had ended, it had taken him minutes before he could silence his own screaming.
"I will grant your desire."
Red hadn't been able to answer. His voice may as well have been stolen from him.
"You will get a second chance."
When he'd come to, it was at the summit of a mountain. He could see Kanto far below, though no matter how hard he thought about it, he could not recall there having ever been a snow-topped peak to the west of the Indigo Plateau.