Haruhiko Endou is a male high school student and a close friend of the protagonist, Junpei Kousaka, attending the same school and often appearing alongside him in various situations. He is characterized by a talkative and inquisitive nature, frequently inserting himself into the affairs of those around him, particularly his friend Junpei. With brown hair and grey eyes, his appearance is that of a typical teenager, often seen in his school uniform.

His primary role in the story is that of a commentator and instigator, especially concerning Junpei's complicated social life. He demonstrates a strong interest in the dynamics of relationships, which he refers to as flags, and actively creates and monitors his own imaginary romantic scenarios involving Junpei and his childhood acquaintance, Kanako Sumiyoshi. He outlines a three-part flag for them, involving a childhood promise, a period of distance during middle school, and a reunion in high school, though he interprets these events differently from the often confrontational reality of their interactions. Later, he also senses a new potential relationship developing between Junpei, Kanako, and another classmate, Kaede Mizuno.

Haruhiko exhibits a cunning and sometimes mischievous side. During a class trip to Kyoto, he cooperates with another student, Kouta, to trick Kaede and Kanako into following Junpei. He also shows a perverted side, such as being involved in an attempt to peep on girls during a class trip. His playful and eccentric personality is further highlighted by his willingness to dress in a maid outfit during a school festival and his habit of cosplaying, most notably as the character Hanamichi Sakuragi from the Slam Dunk series in a school festival event. He is also a fan of certain anime and manga, as evidenced by character keychains attached to his backpack.

Physically, Haruhiko does not possess any supernatural abilities, which sets him apart from the cursed Junpei. His contributions to the plot are mostly social and comedic, often creating or escalating misunderstandings for the protagonist. His relationship with Junpei is that of a typical pair of male friends, with Haruhiko frequently teasing him and offering unsolicited, though often perceptive, commentary on his love life. He has a known interest in twin-tails and displays a somewhat ambiguous sexuality in his mannerisms. While he does not undergo significant character development himself, his consistent observations serve as a narrative tool to highlight the changing and complex relationships among the main characters throughout the series.