(Great, yet another itch until the idea fades three day later, thanks ADHD)
Aoex x BBB AU, ideas dumping
Of course Kuro comes with Rin. Heck that would be the best playground for the cat-sìth. I'd like to think he'll buddy up very fast with Sonic and Leonard, the later probably riding his back more than once with Rin whenever the scooter isn't an option.
As heart wrenching as it is for me to think so, it would be logical for Libra to inform Rin that he's also on watch regarding his level of danger, him being the son of Satan, openly telling him that if he became a threat for people's safety, they would eradicate him. But Klaus would assure him they would beforehand try everything they can to bring him back to reason.
Leading me to totally see Klaus and a couple of other Libra agents training Rin alongside Leonardo. They also keep a close watch on him to make sure he wouldn't get kidnapped by the enemies, because, who wouldn't want Satan's son at their service?
Still, I think Klaus will try to make Rin not feel like a weapon just like how he took care of Leo when they abused the use of his eyes.
Because stories are plain without that one asshole on the mix, Zapp would definitely bully to the poor kid, and even try to fight him. But that was without counting on Klaus and Chain thwarting every attempt since he's a protégé of the Big Guy's acquaintance. In Chain's case, more to piss Zapp off than helping Rin, even if she does care a little about the newbies.
K.K. would smoother him like the mom he never had while Steven and Klaus would be the best Dads.
Rin would definitely part time at Vivian's dinner and save freebies for Leo and Zed while making Zapp pay thrice the price.
Mephisto would occasionally pop in once in a while to check on his little brother, not without wearing earmuffs to ignore Rin's colorful insults. He'd also may or may not keep Rin informed about his "classmates" and brother whereabouts, but the halfling won't want to hear anything from them. Maybe just a very impersonal postcard telling Yukio how much he was thriving now away from his gun, out of spite.
That would make me laugh for Rin, Leo, Klaus and the team to go on vacation in Japan, offered by Mephisto, and being taken aback by the normalcy, now out of Hellsalem's Lot. Well, almost normalcy, since there's still demons roaming around.
Much to Rin's chagrin, when they were outside, he'll have to hide his assets, just like Leonardo will constantly wear his goggles, Klaus wearing a mask and Zed some kind of disguise to hide his fish like traits.
Being a big brother himself, Leonardo will be on the fence regarding the rift betwin the twins, but that won't last long before taking Rin's side whenever the halfling gets on the short end of the stick, much to his and the Libra's team annoyance.
Which lead his old "classmates" and Yukio to see how carefree Rin seems to be when interacting with Libra, like as if being the son of Satan was something totally insignificant like an extra finger.
A very hilarious scene to imagine would be that since they're so used to massive destruction, Libra and Rin getting way overboard during an exorcism of a very big demons or a demon infestation. The True Cross team would be horrified and can't just blame the halfling this time, while Mephisto would be nagging a very sheepish Klaus about the damages done to the place.
"Oh riiiigt! I forgot I wasn't supposed to burn everything on sight here! My bad!" Rin would laugh it off, shocking and riling up his ex teammates.
The local exorcists barely had the time to point their weapons at him and Yukio about to chid him for his recklessness, the Libra team would be in a defensive stance around Rin, pointing their weapons back.
"This boy is a member of Libra. Any attempt on his life won't be without consequences." Klaus warning them, making Rin feel touched.
As the ex teammates are left flabbergasted and some very resentful, Leonardo might approach the group and advise them to be a little more observant before pulling conclusion.
"And what would you know about it?!" Bon would snap at him, only for the young man to open his eyelids a bit. "Let's just say I can see a lot of things. My last name is Watch, after all."