Who is the traitor of ThunderClan?

In which book you mean? In the first book, at the beginning it's Redtail.

Add'l: From the beginning of the series, ThunderClan has had the following deputies: Redtail (Into the Wild; deputy to Bluestar; deceased), Lionheart (Into the Wild; deputy to Bluestar; deceased), Tigerclaw (Into the Wild - Forest of Secrets; deputy to Bluestar; exiled; later ShadowClan leader), Fireheart (Forest of Secrets - The Darkest Hour; deputy and successor of Bluestar), Whitestorm (The Darkest Hour; deceased), Graystripe (The Darkest Hour - Sunset; absent from Dawn - The Sight; deputy to Firestar), and Brambleclaw (Sunset - present; deputy to Firestar).