Assuming that the report that the Royals' pick was traded to the Rangers for Esteban German is correct, that means that Castro, a 20 year old, very small (5'8", 157 lbs.) lefty, is now a member of the Rangers.
If the Rangers want to keep Castro in their organization, they will either have to keep him on their 25 man roster all season, or else send him through waivers, then work out a deal with the White Sox which would allow them to keep Castro.
I don't know much about Castro, other than his stats. He didn't make the Carolina League top 20 prospect list, and wasn't listed in the BA or BP Rule 5 previews.