Bob was a member of Marik's Evil Council. He supposedly created the Millennium Items. After Marik struggles to pronounce "Aknadin" on the 'evil roll call' In Marik's Evil Council of Doom, he decides to simply call him "Bob". Since he only mumbles incoherently, he's not taken seriously.
In Marik's Evil Council of Doom 3, "Steve" Luna claims Bob is a sacrifice that the Island demanded (in reference to the TV series, Lost). Bob was later killed at some point offscreen.