[quote=@Pirouette]
<Snipped quote by Monkeypants>

Your character looks fine to me. The only things I would include are what exactly Lisa has her degrees in, because suddenly dropping she's an experiment in underwater basket weaving when the group most needs one is too unfair. 

Secondly, Billy's degree of disability should be outlined a bit better. Being a marksman with a rifle requires a great degree of concentration and an acute understanding of bullet drop, wind, distance, etc. it isn't impossible but it would unlikely that a mentally challenged person could complete the task with such precision. Not to mention the psychological impacts of shooting a person, failure to protect someone, and so on, would probably weigh a lot heavier on someone who is mentally challenged as opposed to someone who is not. 

Again, Billy isn't an impossible character, but my opinion is that he would have a lesser disability. Perhaps you can include how exactly he is impaired. 
[/quote]

^ What Pirouette said. Fix/Clarify those things, and you're good.