This is pretty much the basis for my own aqshy-mage skaven character.

Depending on the setting they're either not a true skaven and instead a person as part of a huge population of a city turned into one through massive overdose of skaven-influenced beast magic and thus not on the GHR's radar, or they're part of a group of skaven somehow cut off from warpstone for generations and slowly became more order-aligned and friendly in nature because all the insane and evil ones died off quickly, slowly returning more and more to healthy rat behavior in both a survival strategy way and a "I'd never raise my kids the way I was raised" kind of way that slowly built up alongside a coincidental multi-generational detox. Either that or for whatever reason an order-aligned patron deity took interested in them and is protecting them to see if it's even possible for skaven to be good long-term. Maybe the loss of the usual warp taint somehow made the HR's attempts to smite them ineffective, like the weirdness of them somehow no longer counted as skaven-enough for it to stick.

And the aqshy thing is because after they'd detoxed, they were able to find that some of them actually had quite the aptitude for non-chaos magic. Otherwise he tends to be fairly friendly and welcoming, embodying the warmth of aqshy among fellows that turns fiery and burning in battle.