Xero is a lead character but its not told in Xero's POV. Also no one knows what gender Xero is and so its kinda whatever we call him/her at the time. Sometimes we call her a chick, sometimes we call him a dude. It's confusing. 

Something about Xero is that she/he is emotionally rock solid, so there's rarely much emotion other than anger that comes from her/him. She has two fish named Bosui and Oyu, they're her pets.