Learn Italian: È la persona più insopportabile che abbia mai conosciuto. - She is the most unbearable person that I have ever met.

The trigger is the relative superlative phrase la persona più insopportabile (the most unbearable person). In Italian, when you use a relative clause (starting with che) to describe a superlative or an absolute limit, the verb takes the subjunctive. This emphasizes that the statement is a personal opinion or limited to your own experience, rather than an objective universal fact.

Whose perspective is abbia here? Is it "I", "you", or "he/she"?

It's "I" (io). In the present subjunctive, the I, you (singular), and he/she forms of avere are all exactly the same: abbia. Since the sentence means "that I have ever met," the context tells us it's the io form. If it were ambiguous, you could add the pronoun and say che io abbia mai conosciuto.

Why does mai mean "ever" here instead of "never"?

When used without a negative word like non, mai translates to "ever". You will very often see this exact pattern—superlative + che + subjunctive + mai—to mean "the most [X] that someone has ever [Y]". Notice how mai sits perfectly between the helper verb (abbia) and the past participle (conosciuto).

Since the sentence uses la persona, does that mean we are definitely talking about a woman?