I don't have a whole lot of information on them yet, but would love some ideas on how to flesh them out more in terms of the way they would act (I unfortunately don't know as much about aro/ace people than I wish I did, and would welcome any advice on how to write them properly). Please read notes for plot information.
Well, you're right when you say you don't have information on them. You don't know anything besides their appearance. Think about the story and their role in it. What type of development do you want them to have? What reactions do you want people (readers and others in the story) to have towards them? What do they struggle with? Who/what is the antagonist and why are they at odds? What is their motivation (beyond survival, which is very vague) and their backstory? Once you have all of that, it'll be easy to figure out a personality.
Aro/ace and agender characters can always use more representation! Don't be afraid to dig into asexuality.org forum posts or tags on some social sites to get inspiration. :)