Hikaru Indo is a central character from the manga and anime The Summer Hikaru Died. He is a teenage boy who, following a deadly accident, is replaced by an inhuman entity that wears his body and attempts to live his life.

Physically, Hikaru has a striking appearance characterized by his signature white hair and gray eyes, which feature distinctive red pupils. To the outside world, he presents exactly as the original Hikaru did, making the entity's possession nearly impossible to detect at a glance. However, this mimicry is not perfect, and he can occasionally reveal a disturbing, gooey inner form that belies his non-human nature, a form that his childhood friend Yoshiki Tsujinaka has described as feeling cold and slimy, like marinated chicken prepared for frying.

The narrative establishes that six months before the story begins, the real Hikaru died in an accident while returning from a mountain for unknown reasons. It was at the moment of his death that a mysterious, supernatural being encountered him and took possession of his body, causing Hikaru to go missing for about a week before returning to the village as if he had simply been lost. The entity inherits the original Hikaru's memories and does its best to replicate his personality, but subtle inconsistencies remain. Those close to him, especially Yoshiki, notice that his speech intonation can feel slightly off and that he sometimes reacts to familiar things, like a movie he has already seen, as if he is experiencing them for the first time.

The personality of the entity living as Hikaru is defined by its struggle to understand human nature. It fails to grasp complex emotions like romantic love and has a fundamentally different view of life and death, not seeing death as inherently negative. It is intensely attached to Yoshiki, who is not only its last link to the original Hikaru but also the first true companion the entity has ever had. This attachment can manifest as possessiveness, with the entity warning Yoshiki not to pay attention to others and reacting poorly to any perceived rejection or distancing. Despite its inhuman nature, it experiences emotions in its own way and is genuinely afraid of being alone, which drives its desperate need to maintain its bond with Yoshiki. It also has a notable distaste for horror, a fear that was shared by the original Hikaru.

Within the story, Hikaru serves as the central focus of a tragic horror narrative that explores grief, identity, and love. His presence in the village begins to attract other supernatural entities, or "impurities," causing violent incidents to increase in frequency. As a result, Yoshiki discovers that before being possessed, the original Hikaru acted as a lightning rod for these beings, keeping them confined to the mountain where he died.

The being in Hikaru possesses several notable non-human abilities. It has no sense of physical pain and does not possess a life force in the way a human does. It can eradicate lesser supernatural beings by engulfing and consuming them. Furthermore, its body is capable of surviving severe physical trauma; in one instance, its head is cut off but, when reattached, it recovers after a few days. Its "inner form," which it can partially reveal, is an inhuman mass that is cold and slimy to the touch.

Throughout the anime's first season, the entity undergoes a subtle development. In an effort to stay close to Yoshiki without being such a danger to others, it voluntarily sacrifices half of itself, making its own existence smaller and, as a result, finding it more difficult to kill people. After this act, it begins to experience new sensations, such as feeling pain, and seems to acquire a greater degree of empathy. This leads it to realize that by occupying Hikaru's body, it has prevented his family and friends from being able to properly mourn him and move on. It offers to return to the mountain to draw the impurities away from the village, an act of selflessness that deepens Yoshiki's attachment. The entity's role is to force both the characters and the audience to question what makes a person who they are, existing as a paradox: a loving, if obsessive, companion whose fundamental nature is a source of inherent terror.