If you are watching the later seasons, here is a quick guide to their respective 9th episodes:
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
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. If you are watching the later seasons, here
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. It is revealed that Mika Harima , who
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
This episode is often cited by fans as the point where the story "connects the dots" and the main plot finally gains momentum. Other "Episode 9" Variants