It all depends on how lenient the original copyright holder is in allowing likenesses of their characters in others' works. If a heavy hitter like Disney decides to sue you, they can just ruin you with court costs and force you to settle even if you've got a strong case for fair use.
Most of the time you're fine as long as you're prepared to obey a cease and desist order if and when it comes. Best example I can think of for everything working out fine is Howard Tayler changing 'The Seven Habits of Highly Effective pirates' to 'The Seventy Maxims of Maximally Effective Mercenaries' in 2011 (Wikia Link
Use your own judgement. If you think you can get away with it, you can probably get away with it. Just be prepared to change it or get rid of it if somebody tells you to.