Sonoko. She is so annoying. Nothing personal, it's just that I don't like her personality, always acting so stupid.
And the detective boys are annoying, too (Genta and Ayumi). But well, I forgive them, their are just children, it wouldn't be fair to expect more of them.
Also, Shiratori was very annoying when he was interfering in Sato and Takagi's relationship...
And there are some moments when a character I like acts stupid... that REALLY gets me on my nerves, because they are normally not stupid and then I start loosing respect for them...