Amatsu-Mikaboshi ("August Star of Heaven") is the god of evil and the stars in Japanese mythology. The Amatsu-Mikaboshi is not a physical god, but a dark force that existed before the universe. It is a force that feeds off human emotions and can cause them to turn to their darker sides, degrading love into obsession and jealousy, or ambition into greed and powerlust. Its ultimate goal is to dissolve everything back into chaos.
During the King Abaddon crisis, Apollyon, a monstrous locust, was released, one that proved impervious to most forms of weaponry. It was revealed a necessary material for the creation of a sword capable of damaging Apollyon, the Shukukon-Seki, was in possession of Amatsu Mikaboshi, a powerful, rebellious god. At the time, he was imprisoned in the Confinement Chambers' 666th basement. Raidou was forced to ask Kohryu to grant him access to Mikaboshi's prison in order to acquire the needed materials.
Once there, Mikaboshi, the Star of Death, revealed he knew the Gouto-Douji of old, and mulled over his disdain of Yatagarasu. Amused that a new Devil Summoner sought his power, he fought with Raidou and was defeated after a massive display of power on both sides. Mikaboshi, stumped at how he was defeated by a messenger of Yatagarasu's will, gave Raidou the Shukukon-Seki, allowing him to create the Youmetsutou sword, and unlocking Mikaboshi for fusion in the Chaos and Neutral Paths.
If on Chaos or Neutral paths, he will at least gain a measure in comfort in that Raidou does not necessarily obey Yatagarasu blindly, but does his job out of his own will. On a darker note, though, he also reveals he's responsible for the death of another Summoner, implying it was an earlier Raidou.
Mikaboshi fights by continually teleporting around the arena, occasionally stopping to throw either elemental spells or Mute Ray. Mute Ray not only deals considerable damage, but if it connects against a demon, it will be sealed in its tube and won't be allowed to rejoin the fight. Mikaboshi can also create a damaging area with Star's Funeral and deal unblockable damage to the entire arena with Demonic Star. As the fight carries on, he will also start using Dissolution to push Raidou away and prevent close-quarters combat, and combining spells, such as firing Mute Ray while also casting Star's Funeral. While he is weak to Ice and Force, it may be rather hard to hit him as he teleports around.
Amatsu Mikaboshi appears as Naoto Shirogane's new Persona that awakened after Naoto was attacked by Sousei. Unlike Naoto's former Personas, it lacks any combat proficiency due to Naoto's desire to support her friends. When Naoto asked Touko Aoi and Tetsuma Tsuge about the reason why her new Persona is so different from the others she had before, she was told that Personas are representations of the user's psyche, hence the Persona's rather female appearance after Naoto started to embrace her femininity. It knows an exclusive skill named Ability Tune, allowing it to synchronize with the abilities of other Personas. Its Arcana is unknown, as it could be a different Arcana to Naoto's usual Fortune Arcana thanks to the shift in her psyche.