I understand how Dora feels (not the whole secrelty asking a SF out behind your back thing) I too have sufferred from an inferiority complex, and her whole relationship with Marten has given me some insight into her character. I'm willing to bet that the entire reason she went goth in the first place was because of Sven and his actions with her supposed friends (most of whom viewed her as a stepping stone to Sven). She finally figured "here's some friends who won't dump me for my brother."
Of course, the normal activities of these goths eventually became to much for her, plus her growing up and seperating herself from her brother helped her leave the goth phase behind. I can see the slow regrowing of her blonde hair another sign that she's really putting Sven behind her.
Say what you will about Sven, he's been right a few times recently, and is good at standing up for himself, but he's an asshole, nothing will change that, and I can see his "relationship" with Faye going downhill soon if it doesn't change. Maybe the occassional bit of random sex is "healthy" (but I won't start a debate there) but a pure fuck-buddies relationship is not. If anything, this may end up making Faye worse.