Syndra; she is uncompromising, powerful, confident, and confident in her own decisions. Considering Zealot does not necessarily refer to a religion or cult, Syndra's own ideals make her a zealot.
Nasus; his personality and his quotes are dripping with his dedication, resolve (to stop his brother), and toward the balance of life and death.
Kassadin is, without a doubt, the best example of a zealot. He wishes to protect the world from the Void at all costs using the powers he gained it. Malzahar is a zealot on the opposite end of the spectrum which is clear from the fact that he has his own cult which worships the Void.
I realize "fanatic" does not necessarily mean the same thing as "zealot", but they are definitely similar.