Your request to join was declined because your OC has no backstory, no explanation on how she gained her powers, no explanation on what her powers are, and to be quite honest, her ghost half outfit looks too much like Danny's ghost outfit, along with her symbol and her ghost form's last name.
A tip, for her ghost half you might want something that she has around her house like a t-shirt and shorts, avoid hazmat looking suits unless you have a good reason. Also, avoid using skirts as they can reveal too much and could be hard to fight in unless they are short AND they have leggings underneath.
You also might want to work on rounding out her personality. Try giving her a balanced personality with good AND bad traits.
Another suggestion is changing her hobbies/skills/talents.
Again, just being honest here, but you gave her way too much good things she is excellent at. Try limiting her to one or two things instead of a whole bunch of talents. Also, make her be bad at some things as well.
I also suggest changing her top in her human form as well? Again, too much like Danny's top. I would suggest trying out a dark red tank top and a short regular red hoodie with black lining. Just suggesting some stuff, you don't have to take my suggestions but I am trying to help you out here.
One last thing, if she's 14, how long did she have to study Karate before she became a 3rd degree black belt? Just a tip, why not make her a beginner at martial arts?
All in all, a good start for an character ^^ But you still have to work on some stuff.