Red and Blue from the first Generation Pokemon games of the same name. Why?
Consider this: When Red first fights Blue after their incident at the lab, he has a Rattata. Since he keeps it around, it evolves into a Raticate, which Red fights on the SS Anne before fighting Surge. Red doesn't see him for a while before suddenly running into him in the Pokemon Tower in Lavender Town. Where he is facing a lone gravestone. When Red talks to him, he turns and rather angrily asks him what he's doing here, since none of his pokemon look dead. He then asks Red if he know what it feels like. From that battle onward, he never has his Raticate again.
So with that in mind, let's consider something else: Blue's Raticate was severely injured in the fight on the SS Anne, but because of all the confusion and chaos on a major cruiseliner the boy couldn't find it medical care fast enough, and it died. When Red finds him in Lavender Town, he was putting his old departed friend to rest. He never outwardly tells Red that he's responsible, but he does know it. Despite trying to tell himself that it's not Red's fault, that it was just bad luck, he knows it was you that killed his pet, and he will never truly forgive him. But he doesn't harbor an active grudge or attempt to kill him, nothing so childish. That impish kid nature of his shattered long ago. Instead he channels this grief and rage into determination: To fulfill his promise to his departed friend and become the Kanto champion: To finally show Red up and make him face humiliation. Which he does... For all of five minutes, before Red appears and decimates his team again. On top of that, Professor Oak, his only remaining father figure (yeah, he doesn't have any parents) appears, chastises him for 'not caring about his pokemon enough' after all he did for just one of them, and then piles praise and glory on Red. After this Blue is never seen again in the game, and I think we all know why.
Keep in mind that Red is the player character. So throughout the course of a kid's game, you singlehandedly destroy a young boy without laying a finger on him.
