Thank you to jcjace45 for the request of a brother turned into Elsa at a birthday party. Apologies that it took so long and that it came out look a bit wonky. Also I missed the second half of the prompt specifying that it was a voluntary change to begin with, so I will give this another go soon to better fit the request.

The one thing his little sister wanted for her birthday was for him to be part of it. The one thing he wanted was to be anywhere else than a room of kids bouncing off the walls on candy and soda, while Disney songs looped ad nauseum. He was hiding in his room playing video games when his mom poked her head through the door.

"We're bringing the cake out now. She really wishes you were involved."

He sighed and trudged downstairs to the Frozen soundtrack. Just as the candles were blown out, a freezing wind tore through the room like an icy blast. He staggered as frozen fractals spiralled up his limbs. He looked down at his suddenly dainty body while the room dissolved into pandemonium, kids screaming and freaking out.

Only his sister looked unsurprised.

"It worked!" she grinned.

As the last chords of Let It Go faded, things seemed to right themselves. The guests were sent home in tears, and he wrote the whole thing off as a moment of madness. Maybe he'd eaten some undigested crumb of cake, some underdone blot of custard.

Until it happened again. Apparently birthday magic lingered.

He quickly learned to give Disney stores a very wide berth, crossing the street if he had to. He treated it like a medical condition to manage. Christmas shopping could be done in July, and noise-cancelling headphones were a must in public. All to avoid catching even a single note of that song.