The person (subject) who zapped me (relative clause describing the noun person) is (verb 'to be') very nice (adjective describing the subject. i.e The person)
Good example. Your example is a defining relative clause (it defines which person), so we don't need a comma. In a non-defining relative clause, we DO need a comma (E.g. I spend a lot of time on Nostr, which is Bitcoin-powered social media)
