The antonym (opposite) of the word fantasy would be reality, or truth.
Reality
reality
I would say a antonym of guise would probably be "reality", since GUISE is a word for fake.
An antonym or opposite word for myth would be fact. You could also say nonfiction or reality.
If the definition of behaviour is 'appearance or pretense' then the opposite of behaviour is reality, so yes it is an antonym.
reality
The antonym of the word 'synonym' is 'antonym'.
reality,truth
fucjin reality
Fiction is.
Reality.
now ____________ The word when does not have an antonym. Not every word has an antonym.