Given that many anime tackle adult subject matter, it makes sense many series explore mental illness through anime characters with split personalities. Anime characters with DID, or dissociative identity disorder, sport two independent personalities; often this internal turmoil serves as a primary plot line for an anime series. Characters in anime with multiple personalities may all sport clashing personas, but each one gains them in a different way. In order to deal with post-traumatic stress or anxiety, some characters construct a new side of themselves to cope. More fantasy or sci-fi oriented series rely on a a character's "possession" by another spirit or entity, who entwines its own personality with another's.
As is the case with many mentally ill characters, not every single anime character with multiple personalities properly depicts the real struggles of mental illness. Below you'll find different anime characters who deal with split personalities in each episode. Ranging between comedic roles and psychological ones, DID anime characters highlight another way anime explores human nature. It doesn't always get it right, but like split personalities, there's always more than one side to consider.
Rosario + Vampire
Danganronpa: The Animation
Kuroko no Basket
Black Lagoon: The Second Barrage
Yu Yu Hakusho
Mobile Suit Gundam 00
Jubei-Chan: The Ninja Girl