Barry is a human-fish mutant that is briefly mentioned in Maximum Ride Forever. However, he is not seen in any other books.
Before Maximum Ride Forever
When Nino Pierpont first brought the Flock to Paradise, he and Nudge swam at a waterfall along with some of the other mutants.
Maximum Ride Forever
Shortly before Rizal officially takes Max, Nudge, and Total as hostages, Nudge recognizes Barry and waves to him; however, he doesn't wave back.