Eins is the german word for one. He is an aide to Aria Rosenburg and a member of Logos. He died protecting  

Aria from Drei. 


Eins is very protective of Aria and is connected to her. Whenever she was in trouble he helped her. 


Eins has super strength and specialises in close range combat.


In the second season you can see the reason why he is became a member of Logos

In the past, he and Aria were a pair of lovers which all started when Aria gave him a rose. Together they walked on a road and as they approach light, they meet a couple of men, who looks likes thugs. Eins want to protect Aria Rosenburg, so he fights against the men, but a gun-toting thug appears. Aria guards Eins which she was killed for this reason. All of the men ran away. Aria died in the arms of Eins and that was the moment he swore to get stronger.