I'm not sure exactly... *I'm definitely trying to get what you're saying, though...* I THINK what you're saying is that the character, as she's designed right now, stretches the realm of plausibility by being a Muggleborn Slytherin with a "good heart" AND a member of the Order of the Phoenix...?
Also, I remember that we discussed the other consideration before... which is why I added in the beginning regarding the purpose of the character: "when the time is right." I just created her now to keep her from nagging me in my head... and also to serve as a reminder of my interest regarding a Hogwarts position and such (early bird gets the worm and all, I suppose... preemptive strike... whatever you wanna call it... lol). I could just keep her in limbo and not play her until the new Hogwarts school year starts... or... well, I have added a little back-up plan where I can play her as the "master tailor" for Madame Malkin's Robes for All Occasions.
Also... I'm not entirely sure what you mean about her not possessing any of the key values that makes a Slytherin a Slytherin... (And err... not entirely sure what you mean about the Gryffindor part, either, since I wasn't aiming for having her an ex-Gryffindor, and honestly, I think she'd probably fit in that house the least... and I don't think she'd really fit in Ravenclaw, either, since she isn't someone who does a whole lot of studying or learning... Slytherin or Hufflepuff are really the top two houses for her)... She does have ambition (even if she's not very good at trying to achieve her goals), and she has a penchant for breaking the rules (even if she does kind of lack some of the cunning that makes a Slytherin... but then again, so did Crabbe and Goyle)... both of which are Slytherin traits. One does not necessarily have to be full-on evil or mean to be a Slytherin. It's also not impossible for a Muggleborn to be in Slytherin House... it's just very rare.
As for the Order of the Phoenix... well... I suppose that was kind of an after-thought since I recall that you need graduates who are going to be members of factions and such (so, her being Neutral isn't really much of an option...and besides that, I'd like to have a character that fulfills a needed role). As a Muggleborn who isn't as... hmm... twisted as one of my previous characters.... She really wouldn't fit as a Death Eater, and I can see her being very much against pureblood supremacy and such, which would make her a good fit for the Order. The other option is that I could take away her Muggleborn status (making her half-blood) and then tweak her personality and throw her in with the Death Eaters... but that seems like it would probably be a bit boring. (Changing her like that, I mean... I'd definitely like to create a Death Eater character sometime along the line, but this character doesn't really fit that mold).
Maybe there's another option... I suppose I could keep her a Muggleborn Slytherin and an Order of the Phoenix... but make her a great deal more mean, perhaps? As it is, she's already prone to petty and underhanded behavior... It wouldn't be too much trouble to make her a little worse.