She is the daughter of doctor Martinez, and the half sister to Max (found out later in the book series). It is estimated that she is about 12-13 years old. She gives me (personally) major preppy girl vibes but I don't think her voice should be the 'extreme' preppy voice that people normally give. I do think that she would be a bit higher pitched and perhaps younger sounding, despite being thirteen. 

*She probably has the least amount of lines being listed as a main. She is still a character that appears through multiple of the books*