Ail, also known in some adaptations as Alan, is one of the two primary antagonists from the Makai Tree arc of the second season of the original anime. He arrives on Earth alongside his partner, An, as the sole surviving members of an alien race that originated from a sentient entity known as the Doom Tree or Hell Tree. This tree gave life to their people, and in return, it required energy to survive. After their home planet was destroyed in conflicts over the tree's power, Ail and An traveled through space with the tree, eventually landing on Earth to seek a new source of energy to sustain it and themselves.

In his natural alien form, Ail has light green skin, pointed elf-like ears, and long blue hair often accented with pink streaks. He wears a form-fitting dark blue bodysuit. To blend into human society and gather energy, he adopts the human disguise of Seijuro Ginga, a ninth-grade student at the same school attended by the protagonist, Usagi Tsukino. In this form, he has fair skin, neck-length orange hair, and purplish-blue eyes, while retaining a single earring. He and An pose as siblings living in a local apartment, where they secretly house the Doom Tree.

Personality-wise, Ail is portrayed as more outgoing and flirtatious compared to the often irritable An. He is something of a womanizer, seemingly drawing the attention of girls with the music of his flute, and he enjoys the admiration he receives. Despite this, he develops a genuine and sincere romantic attraction to Usagi, which creates a point of conflict with An, who is herself infatuated with Mamoru Chiba. His perception of love is initially distorted and selfish, viewing it as something to be taken forcefully. He also has a competitive streak, particularly against Mamoru, whom he sees as a rival for Usagi's affections. While he can appear calm and sometimes even foolish or awkward when flustered by his crush, he is also inquisitive about human culture, often trying to understand customs that are foreign to him.

The primary motivation for Ail and An is survival. They believe they must drain life energy from humans to feed the ailing Doom Tree, which they think is dying. Ail uses a special flute to summon monsters called Cardians from a deck of cards, and these creatures are sent to drain energy from human victims. Throughout their time on Earth, Ail struggles with his mission as he becomes increasingly fascinated by the positive emotions and energy of the humans around him, particularly Usagi.

In the story, Ail serves as a foil to the Sailor Guardians, representing antagonists who are not purely evil but are desperate and misguided. His role deepens as the arc progresses, shifting from a simple energy thief to a character who learns about love and sacrifice. His key relationship is, of course, with An. Though they bicker constantly and become jealous over their respective human crushes, they share a deep, unacknowledged bond as the last of their kind. He also has a notable, one-sided dynamic with Usagi, whom he tries to win over, and a rivalry with Mamoru and Tuxedo Mask.

The character undergoes significant development. At the climax of the arc, the Doom Tree reveals it does not need ordinary life energy, but rather the energy of love. In the ensuing battle, the tree, driven mad, attacks Ail. An sacrifices herself to save him, dying in his arms. This act of selfless love awakens Ail to the true meaning of love, which he had not understood before. After Sailor Moon purifies the tree, An is revived. Having learned their lesson, Ail and An reconcile, take a sapling of the now-benevolent tree, and leave Earth to start a new life together in space, now as friends of the Sailor Guardians.

In terms of abilities, Ail possesses several powers. He can levitate and fly, as well as hypnotize people by making direct eye contact with them. He is also capable of generating and firing electrical energy blasts as an offensive attack. His signature ability is playing his flute, which allows him to activate and summon Cardians from a special set of cards, which are then sent to do his bidding.