。
A protagonist as utterly ordinary as him is rare.
Literal
He [as-much-as-ほど] utterly-ordinary protagonist [topic-は] rare.
平々凡々 (utterly ordinary) is a 四字熟語 that reduplicates 平凡 for emphasis. たる is the classical attributive form connecting it to the noun 主人公 — a slightly literary touch. ~ほど...珍しい means 'as X as him is rare,' creating an ironic compliment to the character's ordinariness.