耳

How it works

が (subject) marks 耳(みみ) (ear) as the subject, and いい (good) describes it — literally “the ears are good”. Japanese loves this “X is good at sensing” pattern: said of someone, “good ears” means they hear well or have a sharp sense of pitch. No copula is needed because いい (good) is an adjective that can end the sentence on its own.