Fuuka mentions she detests onions which is understandable given that she makes clear her sense of smell is much stronger than a human's, so naturally something stinky like onions would affect her much more than anyone else.
Additionally, her canine features would imply that onions are toxic to her. Dogs can't eat onions, after all.
Reinhard's Bizarre Taste in Food is chuckleworthy until you remember he used to live in poverty before he became a knight. Let's just say he couldn't be exactly picky with food.
