Yasumasa Teratani acts as Ichitaka Seto's primary best friend. He swaggers with confidence in his romantic expertise despite his nerdy look and lack of actual success with girls, positioning himself as a self-proclaimed love guru. This self-perception drives him to offer Ichitaka frequent, often unsolicited advice on romantic pursuits. However, Teratani's guidance typically backfires spectacularly, causing comedic mishaps or worsening situations due to miscalculations, though his intentions remain fundamentally supportive.

Personality-wise, Teratani blends extroversion and bluntness, making frequently lewd or perverted remarks about women, a trait noted by peers and narrative descriptions. His straightforward communication sometimes borders on insensitivity, potentially hurting others without malice. Despite these flaws, he shows unwavering loyalty to Ichitaka, expressing genuine disappointment during Ichitaka's romantic setbacks. His role consistently provides comic relief through failed schemes and exaggerated self-assurance.

Teratani's character development emerges subtly through his romantic subplot with classmate Yuka Morisaki. Initially part of a functional "beta couple" contrasting Ichitaka's awkwardness, Teratani later rejects Yuka when Ichitaka and Iori begin dating. This decision strains their dynamic temporarily, but by the narrative's conclusion, they reconcile and solidify their relationship, mirroring the central romance's resolution. His friendship with Ichitaka also evolves, marked by Ichitaka gradually reducing reliance on Teratani's advice as the story progresses.

In animated adaptations like the OVAs, Teratani's actions align with his manga portrayal. Notably, during a critical incident involving an assault on Iori Yoshizuki, Teratani participates in her rescue. While the manga emphasizes Ichitaka's role, the OVA shifts focus to Teratani leading the intervention, highlighting his underlying bravery and commitment to protecting friends despite his usual comedic persona. His familial background receives minimal exploration, though contextual clues suggest hereditary traits of boastfulness and exaggerated masculinity among male relatives.