Decided to serious post, despite laughing at the Kamina troll attempt. XD
Depends on how you define "mentally challenged". Rei Ayanami, for instance, is obviously challenged in some ways. She's not a fucking retard, but she's clearly got... limits.. to deal with.
Of course, then there's Excel, who is just dumb as all fuck. Learning disability? Check. HAAAIL ILPALA~ZOOOOOOOOOOOOOOOOO *shot for the nth time*