Carla is an unpaid intern and likely to be Carl's sister or cousin since she bears a strong resemblance to him and comes from "a family of unpaid interns", according to her boss Admiral Acronym.
She is the intern of Wanda's Division, in the same way that Carl Karl is the intern of Monogram's Division. She only appears in "Bee Story", where she explains to both Admiral Acronym and Agent Pinky that Professor Poofenplotz is putting up her research over royal jelly in the bee community for her next scheme. She then leaves, announcing that Acronym's coffee is ready.
In "Phineas and Ferb Save Summer", the O.W.C.A. was declared to be shut down because of its budget going over thanks to a ballroom fiasco. However, Major Monogram suggested on Carl's advice that he just create a new O.W.C.A. and fires his superior in order to keep costs down on leadership. It is unknown if Carla is affected by this, either she would be fired because of the original agency shutdown or she would go on to join in Monogram's new agency to keep her job.