“I killed my brother. I won't hesitate to kill you”— Roman to Petra 
Roman is a calculating, cunning person, who is deceiving his friend and has a distinct agenda for his future when he is introduced on the show. While he is first portrayed as at least somewhat affectionate with Petra, albeit a Sin Rostro contact, he is later seen to be violent in nature even towards her and tells Petra much later in Season 1, how he murdered his twin brother in cold blood.
Aaron is introduced later in the show as a jain (hindu non-violent religion), relaxed, friendly and extremely respectful to everyone he comes into contact with – including all living beings such as spiders. Eventually he turns out to have been Roman impersonating his own brother after having faked his own death by murdering the real Aaron Zazo.
Roman is first seen when he introduces himself to the wait staff. He was also having an affair with Petra, a fact that was uncovered by the police when they put eyes on him after he was named by Cortez to be Sin Rostro's contact. After Michael realized who Petra was, he told her to end the affair. She tried to, but Roman grabbed her by the neck so she lied that she was testing him to see if he'd fight for her. She left his room, but the man she thought was Roman was killed before she ever saw him again. 
Roman went to college with Rafael and they roomed together. In some ways, Roman encapsulates what used to be Rafael's behaviour, the difference being that Rafael was running away from a painful childhood and Roman actually becomes a criminal working for a drug lord. The two seem close, as Rafael confides his feelings about wanting to divorce Petra to Roman, showing that Rafael considers Roman a friend, before finding out about his sleeping with Petra and being a murderous, raving lunatic.
Rafael helps who he believes to be Aaron Zazo escape from alleged captivity by paying a ransom (whether that money went to Zazo's plans to snatch Sin Rostro's regime out from under her is never stated, yet very probable).
- Rafael and Roman were college roommates
- Roman's nickname is Zaz
- Aaron and Roman were estranged
- Rafael didn't know the name of Roman's brother.
- It is never known whether the real Aaron was, in fact, a jain and what he did with his life before his criminal twin brother murdered him.