Dazai and Sigma have grown close from the whole “Prison Arc” ordeal. Both of them have injuries from it and have been healing together. 

Dazai is trying to get Sigma to join the Armed Detective Agency since Sigma used to be in the terrorist group, “Decay of the Angel.” Sigma wishes to join them, but hasn’t yet.

Sigma and Dazai are practically polar opposites, but have been getting along.

Sigma has only existed for 3 years, but his physical and mental age is 22. He was made by the “book.”