Jace is pretty easy-going and usually up for an adventure. He is pretty smart but doesn't show it, and has an active sense of humor. Overall he would probably prefer to be running around outside to sitting in the classroom. He'll help anyone who needs help, and is perceptive enough to see when people are upset and aren't asking for the help they need. Still, he likes pulling fun pranks when he thinks he can get away with it. Since childhood, Jace has been practically inseparable from his twin and hasn't even considered that they might be in different Houses at RMI.
With dark blonde hair and blue eyes, Jace looks very similar (although not identical) to his twin. He is of average height and kind of stocky, and is the sort of person who wears shorts regardless of the time of year.
The Fitz family is pretty close-knit, even though their dad is a Muggle. All four kids - Jeremy, Jenna, Jake, and Jake - ended up magical, which their dad took surprisingly in stride. Taking things in stride is kind of a hallmark of the Fitz parents, as they were also pretty calm when Jenna came out as transgender shortly after going to school. Jeremy and Jenna are almost nine years older than Jake and Jace, and the younger two boys consequently don't remember Jenna as anything other than their sister. Although their older siblings went to Southern Magical Academy, the younger twins are attending RMI because Jenna was convinced it was a better environment after meeting Holland at MIT.