Seitarou Yagiri

Seitarou Yagiri (矢霧清太郎, Yagiri Seitarō) is Namie and Seiji's uncle.

Appearance
Little is shown of his appearance in the first season, but he wears a lab coat and has silver hair. In season two, Seitarou has brown hair, like his nephew and niece. He wears a business suit and tie as well as a monocle. His face is round and somewhat wrinkled, and he looks to be slightly older than middle-aged.

Background
In many ways, Seitarou Yagiri is one of the people most responsible for the events that occur in Durarara!! As a premier collector of various strange and supernatural artifacts, Seitarou was the one who sold Saika to Shingen Kishitani, an old acquaintance, to aid in retrieving the head of a Dullahan, which remained his prized possession for years.

When the family business started to fall to the wayside, Seitarou decided to take in his niece and nephew, seeing them as potentially useful tools. Seitarou was far from a loving caretaker, only showing interest when Namie began to excel in her studies. As she grew older, he eventually grew to trust her enough as a subordinate to authorize her as head of research on the Dullahan's head.

Synopsis
After the events of the first arc, Seitarou began working with Nebula; he realized then that they were not in possession of the head, and so he decided to get it for himself. At some point between seasons, Seitarou made contact with Jinnai Yodogiri, to achieve this goal and to get other monsters that Nebula didn't have at that time so he could weild power over them.

In consequence, Jinnai Yodogiri decided that the Awakusu group would be a problem for them and so he tried to weaken them, first using Ruri Hijiribe and then kidnapping Akane Awakusu.

Adabashi Arc
Seitarou goes to Shingen and demands that Celty's head be returned to him. When it becomes apparent that Shingen will refuse no matter what price he is offered, Seitarou threatens Shinra's life in order to get him to comply. Shingen relents and tells him about Izaya Orihara being the last person to possess it. Shingen also warns him about getting involved with Jinnai Yodogiri, explaining that throwing in with that man will only result in being used. Seitarou simply rebuffs that he will use Yodogiri as much as he is using him and ends the conversation.

At the end of the arc, it is revealed that Seitarou and Yodogiri were the ones behind Adabashi's attacks, mainly the one that severely injured Shinra, which was made in order to make Celty lose her mind, however their plan was unsuccessful. Later, Seitarou revealed that his plan didn't consist only of getting Celty's head, but also her body, Ruri Hijiribe and Anri Sonohara. In a flashback, we see Shingen, swearing that he will punch Seitarou for hurting Shinra, but he claimed that he had nothing to do with it.

Final Curtain Arc
He has a breakdown after Celty gets her head back and Shingen threatens to draw on his face.