Initially, it was believed that Baalberith was the one who caused Sytry to fall from Heaven. However, it's later revealed that Sytry was abandoned by his mother during the separation of Heaven and Hell, and Baalberith was the one who rescued him after his wings got burned during an attack. Baalberith sealed his memories in order to Sytry never could remember the abandonment of his mother.
Being a former angel, Sytry seems to be well informed about Heaven's hierarchy and structure.
Sytry appears in chapter 4, after Isaac attempts to summon a demon. He threatens to take Isaac's soul, unless he takes Sytry to the Elector. He then attempts to bribe William into picking him by saying his uncle will support him. After, Sytry reveals himself as a demon, after saving William and Isaac from a demon boar that Isaac summoned. William refuses to elect him, forcing him to kidnap Isaac. Dantalion appears and stops him. The two have a short conversation that is cut short when Dantalion insults Sytry, and Sytry insults Dantalian. William intervenes by cutting off Sytry's power like he did with Dantalion. Sytry has a small flashback of his meeting with Solomon. At the end of the episode, Sytry decides to stay at Williams school, and is promptly punished for going into his (William's) room.
Sytry first appears in episode 2 after Isaac tries to successfully summon a demon (Again). Sytry threatens to take him to the elector or he would eat his (Isaac's) soul. He then attempts to trick William into choosing him by pretending his uncle would sponsor him. Sytry saves William and Isaac from a demon boar Isaac had summoned. He and Dantalion have a minor argument which results in a fight. He also traps William and Isaac to prevent them from escaping. After about 5 minutes, William stops them (The same way he stopped Dantalion in episode 1).
Sytry has a flashback involving his relation to Solomon. Sytry decides to enroll in William's school, the same way Dantalion (and revealed Later Camio) did. Just like in the manga, He is punished by William by washing all the floors for entering his room In episode 3, becoming increasingly popular with their classmates, Dantalion and Sytry continue to enjoy school life at Stradford, much to William's annoyance. At night, William, Isaac, and Sytry heads into the woods to investigate a rumor of a ghost haunting their dormitory. They discovered that it is actually a low-leveled demon under the command of Crosby, who lures them to the church to weaken Sytry's powers in order to kill him. Dantalion arrives and offers to save Sytry if William requests it, who refuses because he does not to rely on Dantalion. At night, Sytry informs William that Maria opened a portal from Hell in order to summon a demon to form a contract with it.