fictional: (adjective) imaginary; characteristic of literary fiction
(e.g. fictional character)
fictitious: (adjective) artificial; fictional, but with intent to conceal or mislead (e.g. fictitious alibi)
fictive: (adjective) relates to a specific fictional element (e.g. fictive kinship)
The meanings of these three words are very similar, and all relate to creations of the imagination. But there are some nuances.
If it’s in a negative context—not being factual for the purpose of being deceptive—use “fictitious”.
If you are discussing literature, “fictional” will probably be your best choice.
“Fictive” is less common than the other two, and is often used in the phrase “fictive kinship” which describes a relationship based not on familial ties but on a close friendship (e.g. when you call your mother’s best friend Aunt Sue).