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TL;DR: Rat is an ex-kittypet-turned-stray from a much more urban area, which the cats called the Place of Tall Nests. He drifted out this way because he wasn't happy there. He's not exactly happy here either and is deeply skeptical re: Warrior Cat shenanigans, StarClan, etc, but he's also a cat struggling with a lack of purpose and something just kinda draws him to the place.
Rat is a cautious fellow accustomed to fairweather friends, and cats that will suddenly leave you high and dry. He's not proud to admit he's also been that cat a few times, it's just how things were. On the bright(?) side, it means it doesn't hurt his feelings too much when it happens.
Left to his own devices, he's cautiously neutral with others until he's sure they're not a threat, then may start to let them in at his own pace. He gets along well with similarly practical types, but he also can't suppress an instinct to make sure cats he sees as more naive/in need of experience don't get too messed up out there while they're still figuring it out. Rat's not a terribly old cat himself, but he's good at taking care of himself and sometimes willing to teach.
The way you avoid getting your tail torn off in the Place of Tall Nests? Don't make anyone want to do the tail tearing. Rat mostly avoids outright conflict unless pushed. That doesn't, however, mean he feels neutral about everybody.
Some easy, fun ways to get on Rat's to-avoid list include: bullying those who are weaker, trying to force him to accept any and all Weird Cat Religions, prying too much, attacking him in any sense (obviously), endangering your cat neighbors by luring the attention of twolegs or dogs into an area, and just generally having not a single ounce of sense in your head (as judged by the biased source that is Rat).
Romance is pretty far off Rat's radar at the moment, given his lack of stability in life and lack of trust in others. It's certainly not out of the question, though. If another can win his affection, they would find a surprisingly loyal and sincere partner in the jaded city cat. The right cat of any gender could catch Rat's interest if things line up well between them and he reaches a point where he's healthy/secure enough to notice.
Kits won't be part of Rat's future unless he acts as an adoptive father, though: he was spayed when he was young and still living with humans.