Hikaru" has short, white hair that is slightly wavy. He has short white eyebrows and gray eyes with red pupils. He has somewhat crooked teeth, most notably a snaggletooth on the upper left. He is 165 cm tall (5'5") and his shoe size is 26 cm. When "Hikaru" gets emotional, his insides leak out. His insides are a swirling, intestinal or organelle-like pattern colored white, black, and gray-brown in the manga and red, purple, blue, and green in the anime.

He is usually seen wearing his school uniform, which consists of a white button up shirt and black pants with a belt, with white sneakers. He also wears a black watch on his right wrist. Otherwise, he typically wears bright-colored casual clothing, such as a T-shirt and shorts. "Hikaru" is carefree and playful. He is highly curious and enthusiastic about experiencing human life. He is expressive with both positive and negative emotions, and at times can lose control of them. These qualities lead Yoshiki to describe him as a somewhat childlike.

Because "Hikaru" has spent most of his existence without emotion or a sense of purpose, now that he inhabits Hikaru's body, he has a strong desire to be accepted. This has led to a very strong attachment to Yoshiki, who provides a sense of belonging to him. He often gets emotional when he realizes he’s hurt Yoshiki in any way. He is very protective of Yoshiki when it comes to other spirits coming near him, making sure to intervene so that they cannot enter him. He can have tunnel vision when it comes to things he wants. In regards to Yoshiki, this at first manifests in feelings of possessiveness and losing control of his emotions when he is afraid of being abandoned by him. Conversely, it also causes him to take Yoshiki's feelings and safety into consideration when he makes decisions, adjusting his behavior so that he doesn't upset or hurt Yoshiki.

"Hikaru" does not experience romantic attraction. In general, "Hikaru" lacks human urges or instincts such as hunger, lust, sexual attraction, and fear of death. He views life and death as essentially the same thing – when someone dies, their soul doesn't disappear, it simply changes form or changes its vessel. Because "Hikaru" does not relate to humans' perspective of life and death, he treats human life lightly and does not instinctually see anything wrong with killing someone if it means protecting his secret. However, even though he doesn't have human instincts, "Hikaru" can still keep factual knowledge about those human instincts. For example, he knows that people get sad when a friend dies, or that certain scenarios are considered "scandalous". He's 270 Jai, don't take this down.