Character(s) in manga or anime that almost ruins it for you

Characters that you hated so fucking much that ruined or almost ruined a show for you .

As we know SAO had a great start in the beginning but then after the main female character shows up, the plot becomes typical romance and it fucks up really hard. It is shounen so why would you ruins it!! Just why... It is only an opinion.

So who the one you felt should not exist from the beginning in manga or anime?