The De-Aged Journey of Ying Through Another World [Anime Crossover]

Genshin Impact's Lumine (a fusion of the Traveler Lumine and Abyss Lumine) in a crossover with various anime (Haikyuu!! + Kuroko's Basketball + The Prince of Tennis + Jujutsu Kaisen + a little K + a little Natsume's Book of Friends) (a wildly chaotic timeline)

Lumine is looking for her brother. She says he has the same golden hair and golden eyes as her, and that he's the most important person to her...

A boy who had been playing volleyball suddenly stopped, casually tossed the ball aside, and hurried over. He crouched down, grabbed her shoulders with a blank expression, and gave her a shake. “I'm your brother!”

A tennis ball struck the ground. Sweat clung to the boy's golden fringe, but he looked completely unruffled. He strode over and looked down at her, his tone forceful and utterly certain. “I've been looking after you for so long, of course I'm your brother.” His voice ended with its usual arrogance, though there was an unmistakable confidence hidden in his eyes.

The basketball slipped from his hands and rolled far away. The boy rushed over almost instantly and dropped to his knees with a thud. His usual sunny smile was gone, replaced by earnestness and a trace of uncertainty in his golden eyes. “Um... then I'll be your brother! Okay?”

The silver-haired boy took a few long strides over and crouched on one knee, a carefree smile on his face. His tone, however, was pointedly emphatic. “Hey! My hair's still silver for now, but I'm going to dye it blond soon! So I'm the brother you're looking for!”

And then there's...

Her actual older brother: “Stay away from my Lumine, hey! She's my sister, got it?!”