Because Durarara!! has multiple seasons split into "cours," there are other "Episode 12s" that occur later in the series timeline:
Confirmed as the founder/leader of the Dollars; contemplating Izaya's advice to evolve. Durarara!! Episode 12
Loses control of Yagiri Pharmaceuticals and her brother’s trust; takes refuge with Izaya. Director : Takahiro Ōmori Screenwriter : Noboru Takagi Studio : Brain's Base Original Story : Ryohgo Narita Notes on Later Seasons Because Durarara
The episode centers on the aftermath of the massive Dollars meeting and the truth behind "the girl with the scar on her neck". Director : Takahiro Ōmori Screenwriter : Noboru Takagi
: It is revealed that the girl Seiji Yagiri loves is not actually Celty's head on a human body. The girl is Mika Harima , a stalker who broke into Seiji's room and saw Celty's head. Seiji, in a panic, attacked her and believed he killed her. To protect her brother, Namie Yagiri had Shinra Kishitani perform plastic surgery on Mika to make her face identical to Celty's.