Toshimitsu Kubo is a disciplined student in Class 2-A at Fumizuki Academy, holding the second-highest academic rank in his year; this rank would become third if Mizuki Himeji were in Class 2-A. He has a younger brother named Yoshimitsu Kubo. Kubo adheres to a strict routine, arriving early every day, including weekends, to study before classes, significantly contributing to his academic standing. Though typically calm, he exhibits distinct nervousness and agitation around Akihisa Yoshii from Class F. He consistently misinterprets Akihisa's actions as romantic advances, mistakenly believing Akihisa is homosexual and interested in him. This misunderstanding evolves into Kubo developing intense romantic feelings for Akihisa.

Kubo readily assists Akihisa to "improve their relationship," except under duress, like lying during a card game to avoid losing clothing. He perceives routine interactions romantically, such as interpreting Akihisa asking to eat dropped bread as a desire for an indirect kiss. In Season 1, Episode 3, he wrote an anonymous love letter to Akihisa without intending delivery; Akihisa received it anonymously, relieving Kubo. Kubo views Mizuki Himeji and Minami Shimada as romantic rivals due to their affections for Akihisa.

In the OVA, Kubo becomes infatuated with a female maid, later revealed to be Akihisa crossdressing. This causes internal conflict, represented by his shoulder angel advocating pursuit of the maid and his shoulder devil urging loyalty to Akihisa. Kubo's reactions here contribute to interpretations of his sexuality as potentially bisexual.

During exam summoning battles, Kubo employs a Shoukanjuu clad in medieval armor with a dark purple hakama, black shoulder pads, and a beaded necklace, wielding dual scythes. He participated in the first battle between Classes 2-A and 2-F, losing to Mizuki Himeji. In later appearances, like "Baka to Test to Shoukanjuu Ja," Kubo frequently visits Class F alongside Shouko Kirishima unannounced, often annoying Yuuji Sakamoto.