"Derek", better known as "Derek the Stunt-Bat", is a retired stunt-bat that was brought by Heinz Doofenshmirtz so he had an easier target for his latest inator. This backfired since Doofenshmirtz just remembered he had a minor fear of bats. Once Derek flew to St. Louis with Doofenshmirtz, Perry used the inator on him again, causing him to return to his normal form.