In season one, not much is shown of him, 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, looking to be slightly older than middle-aged.
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 Namie grew older, he eventually grew to trust her enough as a subordinate to authorize her as head of research on the Dullahan's head.
After the events of the first arc, Seitarou began working with Nebula; he realized then that they were not in the possession of the head and so, he decided to get it for himself. At some point between seasons, Seitarou made contact with Jinnnai Yodogiri, to achieve this goal and to get other monsters that Nebula didn't have at that time so he can get power over them.
Adabashi Arc Edit
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 posses it. Shingen also warns him about getting involved with Yodogiri Jinnai, 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 made Celty lose her mind, however their plan was unsuccessful. Later, Seitarou revealed that his plan didn't consists only on 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.