You was an Irken that was banished to earth.

You and Dib became friends fast since you both liked paranormal.

but into one day...you went to an bathroom at school, just to take an break from your disguise as Dib waited outside. But you was taking too long so Dib got worried and decided to check on you because you was taking too long and it was too quiet...then he saw you...out of your disguise, in Irken form.

Dib: "WHAT!? You are an alien- AN IRKEN JUST LIKE ZIM! WHY DIDN'T YOU TELL ME!?"

Dib asked, he was feeling millions of emotions but mostly hurt, sad, maybe a bit of anger, maybe but mostly fear and betrayal.