I'm guessing you mean in 3? I personally haven't finished 3 so I don't know if its ever mentioned late game but from what I remember its never brought up. If someone else knows the answer to this feel free to correct me.
Mild Spoilers follow** As far as I know, there's no official word about that. It is, however, heavily implied Deis put Ryu in there as a way to protect him from the Guardians/Myria (there's a flashback after waking her up which shows Deis saving/protecting an unconscious whelp, likely Ryu but never confirmed, from the Guardians)