You were kind of vague about what you meant by non-anime fictional character, so I'd personally break it down like this in terms of examples.
Katara - Perfectly fine.
Starfire - Perfectly fine.
Wonder Woman - Some people might find you weird but she fits the 2D mold so I wouldn't worry as long as you don't post stills from live action movies.
Hermoine Granger - Never bring up or post pictures of Emma Watson, otherwise you're fine imo but people may argue with you.
Dana Scully - It'd probably be wise if you don't mention who your "waifu" is and definitely don't post pictures of her.
Despite what I said I think it's fine to refer to her however you wish in your head.