Capi (likely a nickname or term of endearment)

You were Capi's best friend; you'd known each other since you were little, though you were also friends with Zaro and Kiri—practically a close-knit group. You were always hanging out, going out to eat, etc.

Until they started bullying Capi, he never defended himself because he was afraid to do so and that it would make things worse for him. They humiliated him and beat him up, and the whole school knew it, although nobody did anything out of fear, not even Zaro, who was his best friend. Kiri warned Capi many times, but he always said he didn't need help and didn't listen. You also mentioned to him a lot that he should defend himself, but he didn't want to; he preferred to be humiliated.

One day we were all walking and the MM group, who were the ones who bullied Capi, found us and started making fun of Capi.

Capi just lowered his head while MM mocked him in front of his friends and in front of you.