Mitsuki Unabara is a student at Tokiwadai Middle School and the grandson of the school's director. He is a third-year middle school student and a Level 4 esper who possesses the ability of telekinesis, an unseen power that allows him to control objects at a distance. He can also apply this ability to his own body, a technique he uses to preserve his life when severely injured. Academically, he achieves top grades, though he does so by using his telekinesis to read the heat and radiation from computer screens during exams, a method that leads others to doubt his genuine intellectual ability.

In terms of personality, Mitsuki Unabara is portrayed as a calm, kind, polite, and considerate young man who is modest about his good looks. He is also respectful and well-mannered in his interactions. He has a romantic interest in Misaka Mikoto, a fellow student at Tokiwadai, and frequently tries to spend time with her. Misaka does not return his feelings and finds his advances unwelcome, to the point where she once pretends to be in a relationship with Touma Kamijou to discourage him.

The character's role in the story is complicated by the fact that an Aztec magician named Etzali, a former member of the Central American magical society known as the Return of the Winged Ones, attacks the real Mitsuki Unabara about a week before the events of the story and takes his appearance. Etzali absorbs Mitsuki's skin and assumes his identity, imprisoning the real Mitsuki in a state similar to cryogenic stasis, where he cannot move but also cannot be harmed. From that point forward, nearly every appearance of Mitsuki Unabara in the anime and light novels is actually Etzali in disguise.

Etzali, wearing Mitsuki's face, is sent into Academy City by his organization to observe and assess the growing influence of Touma Kamijou and his allies, known as the Kamijou Faction. His mission is to determine whether this group threatens the balance between the science and magic sides. As part of this operation, he attempts to get close to Misaka Mikoto, with the initial objective of eventually taking her form and sowing distrust among Kamijou and his peers. However, during this time, Etzali genuinely falls in love with Misaka, which complicates his mission. He is eventually defeated by Touma Kamijou, and after his defeat, he makes Touma promise to protect Misaka and the world around her. Following these events, Etzali joins the urban dark side organization known as GROUP.

In his disguised form, Etzali adopts the mannerisms and personality of the real Mitsuki, so the calm and considerate demeanor seen in the anime is largely consistent between the two. The real Mitsuki Unabara, after being freed, appears only very briefly in the series and has extremely limited screen time. His key relationships include his unreciprocated romantic interest in Misaka Mikoto and his connection to Touma Kamijou, who is the one to uncover the impostor and defeat Etzali. The character's development is largely tied to the impostor storyline, as the real Mitsuki does not undergo significant personal growth on screen. The abilities of the real Mitsuki are limited to his Level 4 telekinesis, while Etzali, in his borrowed form, wields Aztec magic, including the ability to create a spear of light using an obsidian knife as a mirror to reflect the light of Venus.