Natsuru Seno is the main protagonist of Kämpfer. He is a second-year student at Seitetsu Gakuin High School, an ordinary and somewhat average boy in terms of appearance, grades, and athletic ability. One morning he wakes up transformed into a female after receiving an entrails animal called Harakiri Tora from his classmate and crush, Kaede Sakura. This makes him the only known male to be chosen as a Kämpfer, a fighter compelled to battle other Kämpfer. Because the rule dictates that all Kämpfer must be female, his transformation grants him a feminine form whenever he is in combat mode.

In his daily life, Natsuru is a rather laid-back and timid young man who is often caught off guard by the bizarre events unfolding around him. He holds an intense, single-minded crush on Kaede Sakura and tends to ignore or remain indifferent toward other girls. Despite receiving clear signs of affection from several female characters, including Shizuku Sangou, Mikoto Kondou, and Akane Mishima, he remains oblivious, often rationalizing their behavior in ways that deny the romantic interest. His main motivation is to protect those he cares about, especially Kaede. His discomfort with his own transformation is a recurring trait, particularly when he has to interact with girls while in his female form.

His role in the story is that of a reluctant fighter thrust into a conflict he does not fully understand. He is a Blue Kämpfer classified as a Zauber type, meaning his combat ability centers on magic rather than physical weapons. His primary power is fire manipulation, allowing him to conjure fireballs and other flame-based attacks. His abilities have limitations; he must maintain a balance between his male and female forms, and his emotional state can affect his combat performance. As he fights more battles, his powers grow, especially when driven by strong emotions.

Natsuru’s key relationships shape his journey. Kaede Sakura is his greatest love interest, but the irony is that Kaede develops an intense affection for Natsuru’s female Kämpfer form, leading to confusion and comedic tension for Natsuru. Shizuku Sangou, the student council president, plays a pivotal role in guiding him after his identity is exposed. Following a battle that reveals his female form to the entire school, Shizuku arranges for him to enroll in the girls’ class, which further complicates his social life. Natsuru also forms complex bonds with other Kämpfer, some as rivals and some as allies, and these relationships force him to rethink his understanding of power and connection.

Over the course of the story, Natsuru undergoes significant development. Initially passive and bewildered, he gradually becomes more proactive and learns to accept his dual identity. The exposure of his Kämpfer form to his classmates marks a turning point, altering his social status at school and pushing him toward greater self-awareness. He matures by striving to reconcile the two sides of his life and by taking responsibility for his role as a fighter. His character arc revolves around identity, acceptance, and the struggle to define who he truly is.