Genosai

SaiGenos is the slash ship between Saitama and Genos from the One-Punch Man fandom.

Canon
Saitama and Genos first met when Genos was the about to self-destruct after he was defeated by Mosquito Girl. Saitama however was able to kill her and thus, save Genos. Genos was so impressed by that act that he immediately begged to become Saitama's student. He agreed, not taking that request too seriously.

While Saitama was annoyed by his new disciple at first, he gradually warmed up to him. He also hold respect for Genos as he's an S-Class Hero and also his overall abilities. Saitama doesn't seem too worried about Genos's body safety though, as he's almost completely destroyed in every fight. Even though that is the case, Saitama still takes care of him should be torn to shreds or just saves him from complete destruction.

They're currently roommates and officially teacher and disciple, even though that mostly comes from Genos' side. However, Saitama often gives good advice to him.

Trivia

 * Saitama was shown to be concerned about Genos when he was defeated by the Sea King
 * Genos is seen to be protective over Saitama multiple times