Black Star is a character who appears as a recurring antagonist in the Keroro Gunso manga, first appearing in volume twenty-three. He is a Keronian, identifiable by his small, frog-like stature, but his appearance sets him apart from most other members of his species. His body is primarily black, and he has two long, black pigtails that resemble rabbit ears. He wears blue rings around his wrists, neck, and pigtails, which visually connect him to another character, Shin Keroro. His eyes are shaped like semi-circles with irises that gradient from red to yellow, and a black ring around his pupil gives them a bullseye-like appearance. His symbol is a black star, which is an inverted version of the Keron Star.

Black Star is a mysterious figure, with much of his history remaining unknown. It is speculated that he might be a failed first attempt at creating Shin Keroro. In the story, he is initially imprisoned on an asteroid but is accidentally released by the character Shivava. His powers are said to originate from the Age of Destruction, a dangerous era in Keronian history that predates certain interstellar treaties. He appears to be composed of collected data that gained sentience and materialized, with an independent program serving as the core of his personality and body.

His personality is characterized as wicked, intelligent, calm, and collected. However, he can also become maniacal and brutal. His primary motivation is to steal the Keron Star from Shin Keroro, and he constantly schemes to achieve this goal. His role in the narrative is that of an enigmatic adversary, particularly for Shin Keroro and, by extension, the Keroro Platoon. Upon his release, he easily defeated both Shivava and Keroro himself before ultimately losing to Shin Keroro.

Key relationships define his actions. He sees Shin Keroro as his main enemy and relentlessly attacks him, though this perception is not entirely mutual, as Shin has saved Black Star on multiple occasions and has expressed a desire to be friends. He also recognizes Shivava from an unknown past encounter, describing him as a non-Keronian. When he first defeated Keroro, he complained about why a normal Keroro would possess a Keron Star.

In terms of development, after being critically damaged in his battle against Shin Keroro, he left the planet Earth. Despite this, he continues to appear, often getting himself into trouble from which Shin must rescue him. His abilities are formidable and include dark magic-like powers. He can generate dark energy around his hands, surrounded by tiny runes, to stop time and space. He also demonstrates telekinesis, the power to dispel transformation styles, and the ability to summon shadow-like tentacles. He can communicate telepathically with Shin Keroro and possesses enhanced physical strength, as shown when he stopped Shivava's attack with one bare hand. At one point, he stole Shin Keroro's bracelet, which allowed him to transform into different styles, though without changing his own skin color.