Toshimitsu Kubo ranks as the second-highest academic student in Class 2-A at Fumizuki Academy, surpassed only by Shouko Kirishima. His disciplined and structured lifestyle fuels these high grades, marked by early arrivals at school every day, including weekends, dedicated to study and revision. Kubo typically maintains a calm and collected demeanor, yet interactions with Class 2-F's Akihisa Yoshii trigger significant nervousness and social misinterpretation. He mistakenly believes Akihisa harbors romantic feelings for him, leading Kubo to develop persistent romantic feelings in return. Consequently, he construes Akihisa's mundane actions, like requesting to eat a dropped piece of bread, as romantic advances akin to an indirect kiss.

Kubo penned an anonymous love letter intended for Akihisa without delivery plans; however, Akihisa received it unaware of the sender. This left Kubo relieved his involvement stayed hidden. He perceives Mizuki Himeji and Minami Shimada as rivals for Akihisa's affections, experiencing intense jealousy during Akihisa's interactions with others, though he refrains from physical retaliation. An OVA incident revealed attraction to a female-presenting maid character later disclosed as Akihisa crossdressing, suggesting potential bisexuality. This moment triggered an internal conflict: his shoulder angel advocated for the maid while his shoulder devil urged loyalty to Akihisa.

Kubo's Shoukanjuu summon manifests in medieval-style armor featuring dark purple hakama, black shoulder pads, and a beaded necklace, wielding dual scythes as primary weapons. He has a younger brother named Yoshimitsu Kubo. Kubo occasionally visits Class 2-F alongside Shouko Kirishima, much to class representative Yuuji Sakamoto's annoyance. Across various episodes and media, Kubo consistently displays heightened reactions around Akihisa, including blushing, heavy breathing, and romantic misinterpretations of everyday interactions.