I would actually use two abilities to do this. First I would use Shape Change versus Mental Group so that the basic mind pattern would be disguised. Then I would take levels of Mental Defense with the following; Variable Memories (allowing the person to adjust the memories to fit the situations) (+1/4) and only to set up a false history and memories (-1).
Shape Change (Mental Group, Variable Mind Pattern) 25 points (25 active points)
Endurance Reduction and Imitation can be added for added color.
Mental Defense (+20 def, Variable memories/histories (+1/4), only to give false Memories/ Histories (-1)) 13 points (26 active points)
G