The Collaborative International Dictionary of English v.0.48:
Rebuker \Re*buk"er\ (r[-e]*b[=u]k"[~e]r), n. One who rebukes. [1913 Webster]WordNet (r) 3.0 (2006):
rebuker n 1: someone who finds fault or imputes blame [syn: upbraider, reprover, reproacher, rebuker]