|Gallery | Quotes | Relationship|
|Other Names, etc.||Seki|
|Anime debut||Episode 1|
|Manga debut||Chapter 1|
|Japanese voice||Hiro Shimono|
|English voice||Blake Shepard|
Toshinari Seki (関 俊成 Seki Toshinari), normally referred by his surname Seki, is Rumi Yokoi's classmate and is the cheerful and mysterious titular character of Tonari no Seki-kun. He is always playing games at his desk, from intermediate to incredibly complex, though he never seems to study. In his desk, there is a complex array of well-organized objects arranged in a way that if disturbed by anyone but himself it will be impossible to put back together. He almost never speaks. He has short black hair, green eyes, and is somewhat enigmatic.
Seki seems to have a rather dark personality. He always gives something an advantage over the other. He also seems to think about reality, as he puts his toys in a real life situation most of the time.
Seki also has a triumphant act, as shown in the first episode, when he finally reaches the firework, he plays the fireworks in his mind and looks happy. He usually does this whenever he successfully accomplishes something.
It could be noted that his given name "Toshinari" is made with sounds "To" and "nari" which could make "Tonari" from the series title, Tonari no Seki-kun.