Fans have noted that in the light novel, Celty's head is described as having hair "as black as darkness," whereas the anime depicts it with brown hair.
A series of flashbacks shows that Namie Yagiri has a deep, obsessive love for her brother, Seiji . When Seiji became obsessed with a beautiful head in a jar found in their uncle's study, Namie moved it to Yagiri Pharmaceuticals to keep him away from it. Durarara!! Episode 9
While the anime presents flashbacks from Namie's perspective, the original light novel focuses on Seiji's perspective during these scenes. Fans have noted that in the light novel,
It is revealed that Mika Harima , who had been stalking Seiji, discovered the head in his apartment. In a fit of rage and obsession, Seiji killed her. Namie later had the head (supposedly) surgically attached to Mika’s body. Namie later had the head (supposedly) surgically attached