Mysterykid is one of the most important characters in 13Xs. She has the ability to see into any point in space and time, which was part of an ability Zuka had. Her drawings are meant to be about the past, present, or future events that occur in 13Xs.
She is also a child, hence the name mystery "kid" which explains why her drawings look like a toddler drew them. She is one of the shards of Zuka that Dignity had created when shattering the original body. She is currently deceased, as she faded with the other pieces of Zukember upon overexerting herself killing X.
The pencil is her symbol on the drawing Keys, which is a drawing that Ember designed.