Ken Masters is a fighter from USA whom was sent by his father to master Gouken. There he trained under his Gouken along with Ryu under a fighting style called Ansatsuken. Ken's primary focus is on the Shoryuu principle which is why his Shoryuken move is characterized with flame while Ryu follows the Hadou principle.
The difference between the Ansatsuken twins is mainly in their moves. Ken has a stronger Shoryuken and multi-hitting Tatsumaki Senpuukyaku(Hurricane Kick) while Ryu as a single hit knockdown Hurricane kick and a stronger and faster Hadoken.
I just beat the **** out of you with Ken Masters! PWNED!
Evil Ken is Ken Masters falling or submitting himself into the Satsui no Hadou (Evil Intent or Dark Hadou). There is no sign that Ken has been affected by the Dark Hadou as of yet. Evil Ken is not to be confused with Violent Ken, as Violent Ken was brainwashed by M. Bison(Vega in Japan).
Evil Ken has been given a sort of look in the MUGEN fighting game, made by Reu.