Kōhei Ueno is a supporting character in Mysterious Girlfriend X, where he serves as a classmate and the best friend of the protagonist, Akira Tsubaki. In terms of background, he is a fairly ordinary high school student with an unremarkable appearance and academic standing. He wears glasses and has brown hair. From the winter of his first year, he has been in a secret romantic relationship with Ayuko Oka, a popular girl from a different class. Only a few people in the story, including Tsubaki and later Mikoto Urabe, are aware that they are dating. His personality is generally mild and approachable, but he possesses a notable streak of jealousy and possessiveness, particularly when it comes to Oka. He dislikes the idea of other boys seeing her in a swimsuit and becomes visibly jealous when other boys talk to her. His primary motivation is to maintain and enjoy his relationship with Oka, which provides him with a sense of happiness and stability. He is content in this conventional romance and is often seen sharing affectionate stories about his experiences with Oka. His role in the story is to act as a grounded contrast to the unusual relationship between Tsubaki and Urabe. He provides a sense of normalcy and serves as a sounding board for Tsubaki's romantic anxieties. The stories he tells about his own relationship frequently give Tsubaki ideas for how to approach his own mysterious bond with Urabe. Key relationships include his girlfriend, Ayuko Oka, with whom he shares a warm but sometimes cautious dynamic, and his best friend, Akira Tsubaki, whom he supports while often being left in the dark about the more supernatural aspects of the saliva bond. He also interacts with Mikoto Urabe indirectly through Tsubaki. In terms of development, Ueno's role remains relatively stable throughout the story, though he grows more accustomed to Urabe's presence and the oddities of Tsubaki's behavior. He does not possess any supernatural abilities or special skills, remaining a grounded, ordinary teenager whose value lies in offering perspective.