If you are watching the later seasons, here is a quick guide to their respective 9th episodes:
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. Durarara!! Episode 9
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. If you are watching the later seasons, here
While the anime presents flashbacks from Namie's perspective, the original light novel focuses on Seiji's perspective during these scenes. Namie later had the head (supposedly) surgically attached
Amidst the chaos, the girl escapes and runs into Mikado Ryuugamine , who hides her in his apartment to protect her from Celty. Notable Adaptations & Differences
In Durarara!! , Episode 9—titled —is a pivotal moment where the series begins to tie its many disparate plot threads together. Narrated by Namie Yagiri, it reveals the dark origins of the Yagiri family and the mysterious "head". Key Plot Points