I would say Near from Death Note, or V.V from Code Geass.
They are both annoying characters, who look like little kids and just stand in the way of the MC.
I really would like to punch them in the face.
This is a simple listing topic that predated the forum guideline updates that forbid them. The topic was necrobumped and not noticed for some time it seems. I'm locking it now.
Please use the character pages, your profile, and anime list for ratings and character likes/dislikes. Topics like this tend not to encourage interaction (though there is some) and also prone to spoilers, ranting, and subjective argument.