There are specific storylines within the New 52 Flash series where Wally gets powers. These often involve complex plot developments and encounters with powerful forces.
I have no idea who Wally Purling is in this story. However, if we assume it's a typical Christmas story, Wally Purling could be the protagonist. He could be an ordinary person who has an extraordinary Christmas adventure, or he could be a symbol of the Christmas spirit, spreading joy and kindness during the holiday season. But this is just speculation without knowing the actual story.
Wally and Kuki are likely characters in a particular fictional universe. They could be from a TV show, a video game, or a comic. Without more context, it's hard to say exactly who they are, but they are the focus of the fanfiction, which means fans are interested in exploring their relationship, be it friendship, romance, or some other kind of connection.
Often, the other members of the Young Justice team are involved. Artemis is frequently included. She might be the one trying to rescue Wally or be involved in the events that led to his being trapped. Robin also shows up a lot. He could be using his detective skills to figure out how to free Wally.
Wally could be developed by showing his growth in character. For example, if he was a bit cocky before, he could return more humble and with a deeper understanding of the world. This change could be a result of his experiences while he was away.
Wally could use his super speed to be there for the reader instantly whenever they're feeling the urge to self - harm. His presence alone might be a deterrent.