The character has been subjected to some sort of mind wipe at some point in their past. Unfortunately, the process was somehow incomplete or has not proved successful on the subject. Their old memories are beginning to break through. These memories begin to return in flashes and visions sparked by seemingly innocuous events. Anything might open up the character’s memories, but it will take time for them to put the pieces together.
As a Major Good Trait, the memories are of vital importance. The character is finally remembering events that will be of great use to them and their team. As a Major Bad Trait, the memories are horrific and traumatic, having been removed for the character’s own good.
The character has a memory flash every time they roll a double on any dice roll. When this happens they remember something. If they have the Major Good Trait, the Gamemaster must give them a helpful clue about their current situation. If they have the Major Bad Trait, the character is incapacitated by the horror of the vision for 1D6 rounds. The player and Gamemaster should work together to decide on the nature of the visions and memory loss, and who took the memories away. It should become a sub-plot in the adventure that can be resolved (and the trait removed) as the character rediscovers their past, or takes steps to remove it from their mind once more.