Dissociative Identity Disorder (3.5e Trait)
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Dissociative Identity Disorder
Additional Notes; If the DM asks for an explanation of the differences the best way to explain would be that these personalities are essentially different people inhabiting the same body, and thus will have differences the same as any two people would. However the body would remain the same so the body stats would not change, as such, the rules would remain close to the those for possession. The two personalities would also have different memories which would affect the skills learned. The best way to do this would be to have two different character sheets, roll once for each STR CON and DEX, but seperately for WIS INT and CHA.
Separate Classes?; This would in most cases make sense for both roleplay aspects and game play. The levels would be gained simultaneously, but when choosing the classes be sure to keep in mind armor restrictions. Theres not much point in one personality being a paladin in heavy armor, and her counterpart being a cloth wearing wizard whose spells would be greatly affected as your clothes would remain the same.
Stimulus example; Vin, the half elf rogue is in combat with a dwarf fighter who hits him with a critical strike. Suddenly Vin's demeanor changes, he shifts his stance and he sheathes his offhand dagger, choosing to wield only the rapier in his hand. He has now switched into his duelist personality, changing the course of the battle,and confusing the dwarf just long enough to finish the fight. With the immediate threat gone, Vin changes back with no memory of ever dealing the killing blow but decides that the blood on his sword is sufficient proof that it must have been when he was hit that he stabbed the dwarf, and was dazed.