Athrun

is a mysterious Demon who had saved Tatsumi Oga from Beelzebub when he was oversized. He appears in the manga only.

Appearence
Athrun has long black hair in a ponytail with bangs that hide part of his face. His attire consist of a robe that covers his whole body, clothing similar to the one the thieves wear, and he has a sword in his sash.

Personality
Athrun is shown to be a serious individual who is irritated by Oga's lack of familiarity with his abilities as a contractor. Athrun also seems very cold, as he was ready to kill Beelzebub when he went on a rampage as a giant. He only stopped as Oga managed to return Beelzebub to normal size by telling him it was time to eat.

Plot
He is a powerful demon who appears for the first time in Angelica's house in the demon world. At first, he appears to be working on commission of the thieves' leader as a body guard, however his true mission was to infiltrate the thieves organization in order to recover thirteen stolen treasures.

He saves Oga and the others from being trampled by the giant baby Beel. Athrun states he is disappointed in Oga's inability, as a contractor, to control the rampaging Beel and decides the easiest solution is to kill Beel. Oga manages to stop him by returning Beel back to his original size by telling him it was time to eat.

Athrun thinks to himself about the randomness of Oga's actions and decides not to kill them. Athrun says that they should meet in the future before he teleports away. Athrun is later seen reporting in on the success of his mission to his unknown bosses. Athrun then informs them he met with the Great Demon Lord's youngest son and his contractor and asks if he should have killed them. He is told that it's fine to have let them go since he'll meet them in the human world and for now they should "go along with the Lord of the Fly's little farce".

Power
Athrun appears to be extremely powerful, as he was able to stop the giant baby Beel's foot with only one hand, and being capable of thr﻿owing him without much effort. He is able to teleport himself by using stones that Lamia describes as byproducts of dimensional transfer demons.