Well for this particular YZ plot, each of our characters will have a roman numeral appear on their face before the start of the RP, which leads to them being kidnapped/taken prisoner by Carn. That's how our characters will be meeting up for the first time, so as an Eation user, Ross will have the number "VI" appear, and at some point will be taken prisoner by Carn as a result of this. The method by which this occurs is up to you.
Ross is unique from all the other characters however, in the sense that he is originally from Carn. Naturally, Carn doesn't really take very well to deserters of it's religion, so you can either specify in his history if he's treated different upon capture, or if you'd like to wait until after the RP starts, you can keep that in mind during the first few posts.
The short answer being: If you want to add Carn's special treatment of Ross (A Deserter) into your history, that's your perogative, but do note what happened when his number showed up. You can see an example in my characters' histories, at the very end of each respectively.