He didn't know why for sure this particular little girl had caught his attention, but she had. Perhaps it was because she loved snow and ice as much as he did. Perhaps it was because of her lovely singing voice. Or perhaps, it was because she looked as lonely as he was.
In the short time he had begun watching her, he had never seen her leave her home's premises. She was always confined to the estate, inside and out, but she was never allowed past the gates unless accompanied by a guard. No friends her age were ever brought over to play, and so little Weiss was forced to use her toys as company.
No one could see Jack, so it always gave him the opportunity to sneak into the Schnee estate, and watch little Weiss play with her toys, setting up fake concerts for them to witness. If he were to be honest, it was quite adorable. Whenever she was allowed outside to play, he'd give her some snow, to which she'd play with in delight. She'd make snow angels and snowmen, and would use makeshift hills as slides. Jack always felt a small swell of pride whenever he was able to make her laugh. Even though she'd never see him, the fact that he was still able to give her some bit of company in her lonely childhood made him feel like he was walking on air.
One night, Jack decided to watch over her while she slept. As creepy as that sounded, and as much as that made him out to be a stalker, Jack was doing it to keep watch over her.
Earlier that day, strange black creatures - that had made Jack think of Pitch's nightmares - had attacked the estate, and one had almost killed Weiss. Jack had killed the creature by freezing it to the core and then using his staff to smack it away. Weiss had been alone when it attacked, and she had closed her eyes, so Jack's attack went unnoticed. When guards came to her, they were surprised to find her unharmed. She had told them that it was her guardian angel that protected her, and that had almost made Jack cry. Even though she couldn't see him, she still thought something was with her, and that was as close to a human companion as he would get.
So, Jack decided to stay for the night and keep vigil, just in case those strange creatures (Grimms, from what the adults called them) decided to attack the little girl again. To occupy his time, he had begun playing around with his staff, balancing it on his palm. He moved his hand the wrong way, and the staff tumbled out of his palm. He gripped the end of it, but the hook bonked Weiss on her head. Jack stilled, waiting for her reaction, but the little girl continued to sleep, her nose only twitching a little to show that she felt the hit.
Jack stared in shock as the magic of his staff took over. He knew the staff was magically powered from the Moon, but even Jack had no clue on what it could do. With mild horror but fascination, he watched as the little girl's hair as black as ebony start to turn white as snow. Her tan-ish skin turned pale as well, losing the golden tone her skin used to be. Jack just stared at her in shock and awe before his brain came to a surprising realization: she looked like him. He stumbled backwards at the realization, and stumbled into the dresser behind him, knocking down her hair brush that had been hanging loosely on the edge. The noise of the brush hitting the floor was enough to wake up Weiss, and the little girl blinked her eyes open. Bright blue eyes instead of the auburn brown she used to have were staring at him, even if she couldn't see him.
Her eyes, her hair, her skin...in horror, Jack realized that the staff had somehow, someway, given her similar qualities of himself. He was essentially staring at a female version of himself. Little Weiss, still tired, saw the brush, put two-and-two together, and fell back asleep. Jack was given a few moments to grasp the situation before he fled the scene, leaving her window open behind him.
He never returned back to the Schnee estate. He never saw the reactions Mr. and Mrs. Schnee had towards the change in their daughter's appearance. He never saw little Weiss' reaction to herself. He never saw the joy she had when she found herself with pretty white hair.
He never saw the sad little frown the day she discovered that her guardian angel had abandoned her.