Protagonist
Character Role Analysis
Pnin
In case you had any doubt that Pnin is the protagonist, take a glance at the title. Yeah, it's his name. Duh.
Okay, okay. We know that's a bad reason to claim his protagonist-hood because "The Legend of Zelda" is named after Princess who is trapped for the majority of every single version of the game. Just to pick the most obvious and literary of references. But it's still a pretty good indicator.
Okay, so more reasons. Pnin is the one character who appears in every single chapter. We know his thoughts, dreams, hopes, and even his secret loves. We might not cheer for him, but we at least feel sorry for him when bad things happen. And we might not experience the world through his eyes, but we are constantly peeping into his life. Anyway, no other character sticks around long enough to snatch away the title of protagonist from our plump, spindly-legged old man.