Kubo is a student of Yankumi's class of 3D in Shirokin Academy. Like but a few other students, he appears often initially but is never focused on in the main storylines. He is often seen mumbling to himself as he is when Yankumi first entered the classroom. He was one of the only two students inside and because of it, appears to not share the same interest in females, as the rest of his classmates.
His personality appears to be quiet but unpleasant, causally muttering curses but never out loud. In chapter 1, he is also the student who initially threw the pachinko ball at Yankumi but appears as shocked as the rest when she catches it. His name is mentioned in chapter 14 when his score is the second and only scrore announced, behind Sawada Shin. He is still drawn as mumbling beforehand but appears pleasantly surprised by the result.