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Two months after Kaeya shows up at the winery and one week after Crepus decides to formally adopt him, Diluc drags Kaeya out to meet Jean so he can show off his new little brother and inform Jean she can’t say she’s in charge because she’s the big sister anymore.
Almost as soon as the introductions are done, Jean and Diluc run off to see how much Jean’s pet tortoise has grown - a whole centimeter and a half since the last time Diluc visited! - leaving Kaeya alone on a strange estate with no idea if he’s supposed to follow or wait here for Diluc or go back to Master Crepus.
While he’s trying to figure out what Diluc expects him to do, a little girl skips up with a sunny smile and a missing front tooth.
“Diluc said you’re his little brother now,” she informs him. Kaeya nods. “I’m Jean’s little sister.”
“That’s a weird name, Jean’s Little Sister.”
“My name’s not Jean’s Little Sister, it’s Barbara!” she says, stamping one foot and pouting.
Kaeya laughs. “My name’s Kaeya.”
“It’s nice to meet you, Kaeya. I bet since you just became a little brother, you don’t really know how to be one yet, huh?”
Kaeya shakes his head, because he really doesn’t. All anyone warned him about or taught him to do was for the things Master Crepus might expect of him; no one ever told him anything about what’s expected of a little brother.
“I can teach you!” Barbara says brightly. “I can be your big sister!”
“I’m pretty sure I’m older than you.”
“Yeah, but you’ve only been a little brother for a little bit, and I’ve been a little sister my whole entire life, which means I’ve got see… see… seen...”
“Seniority?”
“Yes, that! I’ve got senior tea! Don’t worry, I’ll teach you all about being a little sib!” She grabs his hand and starts tugging him off in the direction Diluc and Jean went. “Your first lesson is never ever ever let your big sib think they can just go off without you. Come on!”
Kaeya lets himself be pulled, and lets himself smile, and decides he likes Barbara. She can be his first friend besides Diluc (if Diluc counts; he’s not sure if brothers count for this sort of thing). And if she turns out to be his only friend besides Diluc, that’s okay.
Too many friends might make his job harder, anyway.