"He's My Only Vampire" is a supernatural romance manga that intertwines themes of love, mystery, and the supernatural. The narrative centers around Aki, a high school girl who becomes entangled in the world of vampires after a chance encounter with Kana, a mysterious and alluring vampire. Kana saves Aki from a life-threatening situation, but this act binds them together in unexpected ways. Kana, who is centuries old, reveals that he has been searching for a specific girl who can provide him with the blood he needs to survive, and Aki appears to be that girl.

As the story progresses, Aki learns more about Kana's past and the complex dynamics of the vampire world. Kana is not the only vampire with an interest in Aki; other vampires, including Kana's rival, Jin, also seek her out for her unique blood. This creates tension and conflict, as Aki must navigate her growing feelings for Kana while dealing with the dangers posed by other vampires. The relationship between Aki and Kana evolves from one of necessity to a deeper emotional connection, though it is fraught with challenges due to their differing natures and the external threats they face.

The manga explores the moral and ethical dilemmas of immortality and the consequences of living as a vampire. Kana's struggle with his own nature and his desire to protect Aki from the darker aspects of his world add depth to his character. Aki, on the other hand, grapples with her own humanity and the sacrifices she must make to stay by Kana's side. The story is punctuated by moments of action, as the characters confront rival vampires and other supernatural entities, as well as quieter, introspective scenes that delve into their emotional states.

Supporting characters, such as Aki's friends and other vampires, play significant roles in advancing the plot and providing additional layers to the narrative. The manga balances its romantic elements with a broader exploration of the vampire mythos, creating a story that is both engaging and multifaceted. The interplay between the human and vampire worlds, along with the personal struggles of the main characters, drives the narrative forward, culminating in a series of events that test their bond and resolve.