Schrödinger's Trap

When you combine Schrödinger's cat with anime Traps.

Schrödinger's Trap is a person that unless directly asked you cannot be certain of their sex as it can change as the story progresses.

"Is Ruka Urushibara from Steins;Gate a guy or a chick?"
"Both and neither, a Schrödinger's Trap. Rintarou fell for it."

or SPOILER for Tengoku Daimakyou

"Have you seen Tengoku Daimakyou? Kiruko's a flippin' dude inside!"
"Yeah, never would have guessed it, another Schrödinger's Trap."