Gren was the sort of person who needed others; everything he was and did was shaped by those around him. He made some good choices and some bad, but these are the people that were important to him.
Gren met Faye at Rester House where he was performing, and after he rescued her from a gang of thugs - a move which was both unwelcome and unnecessary - he took her back to his apartment. At first he just provided someone for her to talk to, listening to all her problems and offering what advice he could, but after she found out his secret their roles reversed and she became the second person after Julia that he could share his whole story with.
Some people try to make an argument for romantic or sexual attraction between the two of them, but I don't think that was the case. Although there might have been an initial attraction on her part, Faye came on to Gren because that's how she deals with men. As for Gren, right from the start he made it clear he wasn't interested in her in that way. What did happen between them was an odd sort of friendship born of circumstance that left them both a little better off than they were before. Through her time spent with Gren, Faye was forced to accept her need for other people. And Gren had one last brief moment of companionship - a small measure of comfort before what he knew was inevitable.
A few years ago Julia spent three months in Blue Crow and often frequented the bar where Gren worked. Gren and Julia became quite close during that time, and she became the first person that Gren opened up to and told his whole story. Julia could also be seen as one of the triggers for his final act of revenge against Vicious, as she knew enough to put the pieces of the story together and forced Gren to realize without a doubt that he had been betrayed.
Gren idolized Vicious, and it would not be a far stretch to say that he loved him. At the very least Gren had a serious crush on the man. Although it appeared Vicious felt something back for him, knowing the man it was all manipulation on his part. In the end his gift to Gren - a music box - was a way to get rid of evidence hidden inside. He then turned Gren over to the authorities with no remorse for his actions that left both an emotional and physical mark on Gren. Once a happy young man, Gren now lived only for revenge because Vicious had taken everything else away. The moral of the story: Vicious is not boyfriend material.
Gren fought in the same unit as Vicious in the Titan war, although it's never said exactly what they were fighting for. Well, it can easily be assumed that Vicious was there as a member of the Red Dragons and working as a spy. So while Gren saw Vicious as a comrade who was there for him in a dangerous time, Vicious saw Gren as someone who would take the fall.
Spike and Gren didn't really spend a lot of time together, and most of that was in combat with Vicious. In fact, the only reason they helped each other out at first was probably because they were both after the same man, and Spike was actually after Gren's bounty. But when Gren was mortally wounded and Spike ran to his aid, Gren recognized who Spike was from his two-coloured eyes and was able to give Spike what he sought: information on Julia. In return, Spike helped Gren to die with dignity en route to Titan. So although their meeting was brief, they both had an impact on each other's lives.