Hastaa is a character from the anime series Kamitsubaki City Under Construction. Hastaa is identified as a special kind of Tesseractor, which are beings central to the conflicts within the city's narrative. Physically, Hastaa has a distinctive and striking appearance, taking the form of a small, red fish. In humanoid depictions, Hastaa is presented as androgynous with a youthful appearance, typically shown with a bowl-cut black hairstyle that reaches the ears and a single visible blue eye, often partially obscured by a hoodie.

Within the story, Hastaa plays a notable antagonistic role, being infamous within Kamitsubaki City as a serial killer. Their specific targets are the Singing Women, individuals who are vital to the city's survival and a core part of its defense system. Because of these actions, Hastaa has earned the nickname Crybaby.

In terms of personality, Hastaa is known for being overbearing and provocative. Despite these intimidating and formidable traits, which also make them a clever and dangerous opponent in battle, there is a lingering childishness to their character. This is reflected in a quote attributed to them: Do what you want.

Hastaa possesses extraordinary and unique abilities as a Tesseractor. Their primary power is the ability to create copies of themselves. This ability makes them particularly formidable in combat and difficult to counter. They are described as a subspecies similar to another character, Laplus, which distinguishes them from other Tesseractors. A key relationship noted is a direct address to a character named Rime, with whom they share a significant connection.

Over the course of the series, Hastaa remains a persistent threat due to their targeting of the Singing Women, directly opposing the main characters and forcing them to confront a dangerous and relentless enemy. Their actions and motivations are central to the dangers that plague the city's inhabitants.