The Pirate Queen
Although Omega has no actual ruler, Aria T'Loak claimed the position for herself through sheer cunning and brute force. As she herself says, Omega has only one rule: Don't fuck with Aria.
After having left the turian military, Nyreen made her way through the galaxy as a mercenary and eventually came to Omega. It was here that she encountered Aria for the first time. Nyreen was immediately taken in by Aria's beauty and her personality; specifically becoming mesmerised by the fact that Aria never let anyone else define her identity. The turian and the asari quickly formed a relationship with each other.
"But then there was this beautiful, fierce creature who didn't let anyone define her. I'll admit, I was mesmerised."- Nyreen, talking to Shepard
While her own people shunned her for her biotic abilities, Nyreen found acceptance and even appreciation in Aria. As a powerful biotic herself, Aria spent time helping Nyreen learn how to hone her own biotic abilities in order to use them to their full potential. Aria was integral to Nyreen's ability to cope with no longer being a part of the turian military, helping her find purpose in life. Nyreen would later reflect that Aria had all of the strengths that she wished she possessed herself at the time... Aria was truly something to behold.
Eventually, Nyreen began to feel that the more time she spent with Aria, the more she was trying to become like her and her own personal identity was starting to vanish. It was at this point that Nyreen decided she had to end their relationship, a decision that naturally did not go over well with the asari. All we know about the breakup is that Nyreen left on a fairly angry note (likely due to Aria's volatile lifestyle) but that it was still very hard for her to break away from Aria.
Aria believed that Nyreen had left Omega for good, but Nyreen stayed behind, unable to leave. We're never really sure if she was unable to leave because of Aria or because she felt that she had to help the civilians of Omega... either way, the two would not meet again until Cerberus' occupation of the station. When they do meet, Aria is civil towards the turian but she's also immediately suspicious of Nyreen's presence on Omega. Nyreen is somewhat more hostile, being quite vocal in her dislike of Aria's violent tactics; she openly dislikes Aria's blatant disregard for the civilians of the station.
Although the tension between the two is quite high initially, they seem to gradually ease up on each other. Nyreen confesses to Commander Shepard that she's actually quite grateful to Aria for reminding her of who she really is and that her efforts to protect the people of Omega are not intended to undermine Aria in any way. Aria attempts to help Nyreen get over her fear of adjutants by giving her a bit of tough love; Nyreen convinces Aria that it was right for Shepard to attempt to reroute power in order to save innocent civilians, rather than take the fast way out. They seem to be able to get through to one another and Aria even flirts with Nyreen closer to the end of the mission.
"For all the drama between us, I'm grateful to Aria. She helped me remember who I am. It took a lot to break away from her... but I regret nothing."- Nyreen, talking to Shepard
When Nyreen sacrifices herself in the end, Aria is absolutely livid; she loses all control and goes on a full assault against the General. Obviously Aria still harboured some feelings towards Nyreen because of their history; Aria isn't the type to become overly attached and trusting to just anyone. Their relationship, despite being doomed, was obviously a very strong one; it's sad that we no longer have a chance to see how this relationship would play out in the future, but it would be nice to see material related to their relationship in the past.