Have her be naked

"Lucy," Natsu ordered. "Towel off."

"Fiiinne," Lucy huffed. She began to unwrap herself, revealing to Natsu her large tits and clean shaven pussy. "You're lucky you have me as a spirit for a bit, otherwise I wouldn't be doing this."

Natsu didn't like Lucy's attitude, so he made a wish to have any woman he summoned with the keys love obeying him and also be a bit more sexual.

Lucy gained a sultry look on her face and also posed in a seductive manner. "Like what you see, Master Natsu?"

"Oh yeah," Natsu said.