Haha, Merry late Christmas. :)
This is from Krysten Malfoy's story, The Truth.
I got her permission to do this, so don't worry, I'm not stealing anything. :P

I started glancing around the store, looking out for agents. That's when I noticed a familiar man in a lab coat. Ferb noticed me looking and followed my gaze. He tapped my shoulder and pointed, an eyebrow raised.

"That's Doofenschmirtz. He's my nemesis." The green-haired boy tilted his head, confused for a moment. Then he gave me a nod. He blinked, then looked behind me. I turned around as well to see Doofenschmirtz standing there.

"Um, yes, have you seen a blue platypus with a little brown hat anywhere?" I just raised one of my eyebrows at him, confused on why he was asking random people.

"Doofenschmirtz, it's me." The evil scientist looked at me like I was crazy.
"No, no no no, Perry the Platypus is, well, a platypus." I grabbed him by his shirt and pulled him closer, a threatening look on my face.

"Listen, Doofenschmirtz, I'm not sure how I can prove it, but I'm really Perry the Platypus. Your nemesis." He scoffed.

"Well, if you can prove it, then I'll believe you, I guess..." Thinking for a moment, I finally decided what to do.

. His jaw dropped and his eyes widened.

"Perry the Platypus?! I mean, uh... Perry the Teenage Girl?!" I rolled my eyes.

"Yes, Doofenschmirtz. We've established that. Now, I'm on the run from Monogram, and you can help me by not knowing we're here or you could turn me in and possibly loose me forever. Your choice." I crossed my arms. Then thought for a minute. "And it's Agent K to you." The evil scientist sighed, then looked down at me, an annoyed expression on his face.

"You have some nerve, Perry the no-longer-Platypus. Some nerve."

"Agent K, Dr. D." He rolled his eyes.

"Whatever!" Ferb tapped me on the shoulder. "Who's this boy, Agent K-slash-Perry-the-Platypus? Someone I should know about?"

"Dr. Doofenschmirtz, Ferb. Ferb, Dr. Doofenschmirtz." They both just blinked at each other, not saying anything.

"Well, goodbye, Perry the Teenage Girl. I must go build a new -inator... feel free to come by if you wanna stop me or anything..." He simply walked off, waving his hands nonchalantly. Ferb just blinked, then pointed to the other side of the store at Phineas and Candace. The red-haired boy saw us and walked over.

"Oh, there you are Perry... I mean, uh, Krysten."