Celty's head

In the opening sequence of Durarara (デュラララ！！) , you would notice that only this woman does not have a name, but yet appear as an important character.

Enri Sunohara shows her diary of her with Harima, a friend from middle school who had gone missing before school began. Sunohara mentioned that Mika Harima was stalking Seiji Yagari, the guy in the second episode who came late only to declare that he won't be coming to school for "something more important". Notice that Mika Harima likes to wear a hat and clothes that are white and pink.

When I first saw this in episode 2, I didn't realize that there were two empty seats instead of just one right behind Mikado Ryuugami. The other one is in the red circle of the right image.

Sunohara became the class representative as she and Harima wanted to become one. She looked at the empty desk where she should have sat. Since Ryuugami was sitting right in front the empty desk and she was looking for a male class representative (as indirectly mentioned by the old man), her looking at that direction made him raise up his hand.

Who is this woman that has an odd scar that Ryuugami bumped into episode 1 that later became Yagiri Seiji's lover? Notice that the colour of the skin is diifferent. I would assume that this was just after the head transfusion. Well, how do you explain the odd scar mark and the uneven skin tone? The mention of Celty's head gone missing in episode 4 and the head shown in episode six must mean something, though it is revealed that the kidnappers seen in the first two episodes is actually (one of) the supplier(s) for the Yagiri Pharmaceutical.

Here's a theory of what might have happened: The Yagiri Pharmaceutical company (managed by Seiji's older sister) hunts for people that wouldn't be noticed if they had went missing. Yagiri Seiji likes Celty's head, but Mika Harima likes him. Mika Harima might have heard (after stalking Yagiri) that the company does experiments and have somehow acquired Celity's head. So to have Yagiri love Mika, she decides to have her head swapped with that of Celity. Just after the surgery, she decides to have a walk around with her new head. She bumps into someone (Ryuugami), thinking that it's Yagiri, but upon realization that it's someone else, she runs away in horror. (This was the only time we heard her voice) She searches high and low around Ikebukuro, but eventually, she found him.

Speaking of episode 6, this episode revealed a lot of the mysteries from the earlier and made me look back at the earlier episodes and would notice details that I wouldn't notice.