"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.
In his normal form, he looks like a normal bat.
In giant form, his appearance gains a more ferocious and dangerous look: his teeth get sharper and larger and his eyes become narrower.