Meant to be a one-shot, may have follow up side stories. Cross posted on AO3
Even with all their practice as Paris' first and foremost superheroes, the one thing they weren't prepared for promptly hit them with all the force of an angry gorilla's deliverance of a concussion and two regretful kwami. They were told nothing of actually taking on characteristics of the animals their respective miraculous represented, hence the regretful, slightly embarrassed kwami.
Marinette noticed first, though her symptoms as Ladybug were much more subtle, yet still noticeable. She found out that ladybugs were naturally a calm species, which in the long run helped with her endeavors of admiring (stalking) Adrien Agreste. She also learned that cold weather was a deal breaker, and while she had never really liked being cold in the first place, it was almost unbearable now. This, of course, made it harder than usual to go out in the winter to save Paris from the latest threat of Hawkmoth, or even a simple social gathering.
Adrien, on the other hand, was very concerned when he felt the sudden need for a good back rub 24/7, or the fact that he apparently disliked water now. He quickly learned that cats are standoffish and sometimes downright rude when just woken up (but that was a story for another day; the class laughing behind him be damned).
Both Miraculous holders complained to their kwami, and the kwami hesitantly explained and complained back. Marinette could live with it, accept it even, though the whole cold thing was still absolutely terrible. Adrien still desperately wanted a nice back rub (and like hell he was going to ask his father, Nathalie, or Nino for one).
After deliberating for some time, he transforming into Chat Noir, simultaneously jumping out the window into the dark night, going to a person he knew was kind enough to help. While she didn't know his identity, Marinette was the obvious choice. She was already acquainted with Chat Noir, for one, and she seemed to have an affinity for helping people.
Moments later, he came to the roof of the bakery/apartment her parents owned, knocking on the hatch to the room he knew was hers.
Marinette was alerted to his presence, though she thought it was strange that someone would be knocking on her roof. She peeked out the hatch to see who she considered her other best friend. "Uhh…" she said intelligently. "Hi?"
She received a wave in response, something she also thought strange. Chat seemed on edge, almost antsy in a way she couldn't explain. Chat entered the room.
"So what brings you to my room this late at night?" Marinette asked. She was curious about what warranted a visit from her feline outfitted friend.
"Pet me," he demanded simply.
"I'm sorry," she replied, "what?" It wasn't so much an incredulous or confused look she gave him as much as it was an amused one.
"I can't take it anymore!" He sounded frantic. "The Miraculous has been all weird and basically I'm more like a cat now and I need a back rub or something! Please!" Marinette swore he didn't breathe throughout that whole sentence.
"And you came to me why?"
"Because you help people?" A sheepish look now adorned the part of his face she could see.
A small smile crossed her face as she chuckled. She sat on her bed and pat the space beside her, saying "Come here."
He complied almost instantly, trudging over to the bed and sitting with an unceremonious flop. His head found its way to her lap, and she chuckled once again, running her hands through his hair.
A low grumble forced itself from his throat, and she abruptly stopped. "Chat," she said. "You're purring."
"Th-that's new…" he stuttered out, his cheeks going a few shades darker.
"It's cute," she said with a giggle. This did nothing to help his blush. She resumed, and the purr continued.
Needless to say, this became a recurring thing.