Sure. I’ll try to recall the events from the top of my head. It’s a bit of a lengthy read.
Diabel is a former human. When the fl asked why he couldn’t just use his devil powers in the previous timeline, this is because he hasn’t turned yet. His transformation was when he showed himself to the fl in the prison cell, night before her execution. Basically, human Diable is his real self and devil Diable is just an act (an imitation of the true devil, Tara). His contract will end after the fl sees through her revenge (Erez will resurrect him soon after and reunite with the fl).
Erez also made a contract with the devil in the first timeline and somehow knew about the duke’s assassination attempt to fl’s family (he ordered the bandits to spare the fl, stating that the duke has other uses for her). He’s also seen to be visiting the fl’s cell often and negotiated a way for her release which the fl thought was his way of mocking her fate. In the second timeline, Erez will murder the duke and his older brother in the future and sabotage his sister’s plans. Driven mad by this, he threatens the fl to be with him but gave up soon after seeing the fl’s mental breakdown. Tara, disgusted by Erez’s show of ‘love’, ordered him to kill the fl in which he refused.
After the fl and ml started their own family, Erez has made some appearances in the form of a fox. Tara asked why he resurrected the ml instead of having the fl for himself, he replied that human emotions are fickle and he will be by fl’s side when the fl’s or ml’s love runs out.
All in all, Diabel is the pure hearted type while Erez is the dark hearted type (reverse roles).
Erez seems to be the popular choice according to knetz