We know at least this much:
1. Homura is not from the present timeline.
2. Homura apparently can't tell Madoka or the other girls outright a. that magical girls' bodies are shells and their soul is in the gem and b. that magical girls turn into witches. That would have been the easiest way to keep them from doing it.
3. She claimed to have tried to tell other girls the same and they just didn't believe her. Meaning either she's lying or she is an anti-magical girl activist for the whole community.
3. She wasn't contracted by QB himself (since he didn't know her power til ep 8 ) or she was but when she went back in time QB didn't know about her anymore.
There's a lot of speculah about Homura's origins - that she was the 'original' magical girl and has since been trying to save them all, that she was helped by Madoka in the future and went back to the past to save her from becoming a MS, that she's the cat from the OP somehow, etc.
I was riding the 'saved by Madoka' train for a while until I realized: Homura doesn't necessarily have to care about Madoka in particular to be watching her - she just has to have the foreknowledge somehow that Madoka has the potential to turn into an unstoppable universe-ending witch. Keeping stresses off Madoka and going to great lengths to make sure she doesn't contract isn't really indicative of a relationship, just that Homura doesn't want the world to fucking cave in on itself. However, ep 8 really showed that Homura cares about Madoka... I think... BUT that doesn't mean the show will take that route, because it likes to do what everyone is not expecting and is pretty good at it.
There are potentially a lot of surprises left: Madoka never becoming a MS is one I'm hoping for, cause it makes the OP even more hilariously wrong than it already is. QB directly trying to force Madoka to contract or basically turning aggressive somehow other than in recruiting would be pretty sweet. Madoka using her wish to do something crazy awesome (bring back Mami and Sayaka, or all the MS who ever turned into witches, or stop the whole operation, or ... there are so many game-breaking wishes though). Can't waaaaiiiitt.