I guess this is my responsibility, heh. Anyways...
Some reason anyone likes any character. It doesn't necessarily need a justifiable reason and it's not always the same between fans.
For myself though, I've always been a Star Fox fan but I'm actually one of those people that really loved Adventures and Assault. Back before Brawl I wanted another Star Fox rep and my top choices were Wolf and Krystal, though I supported Krystal a bit more due to the moveset potential I saw. When Wolf got in, Krystal was obviously next in line in my eyes. I started supporting Krystal in this game well before Smash 4 came out just assuming Wolf would come back. He didn't, so then I felt I had to support both.
Specifically, I like Krystal because she represents Adventures and Assault and though her character isn't really well developed, she still has all the foundation pieces to being a really interesting character. She's likable enough for me anyways, though that's subjective and where a lot of people are split. In terms of her moveset and stuff, well, I'm sure you've heard the staff mentioned before. That easily realizable potentially unique moveset just makes her stand out a bit, especially relative to her Star Fox cohorts. Plus, when I think about the staff's abilities and Krystal's abilities to use it as a physical weapon, I just believe I'd immediately love that moveset.
There's lots of other characters I support too but a mixture of my long time interest, her potential uniqueness, her decent chance, and my care for her franchise and Smash bros too has lead me to be 100% behind her inclusion. If she was already in the game, I could see myself being in Isaac's corner for instance for similar reasons though I haven't played Golden Sun in years.