I don't say contrarian isn't a valid answer, but it's not that common. People are more likely to say...
"He's just being contrary" rather than "He's a contrarian"
But far more common in my experience (particularly in speech) are the idiomatic...
"He's a dog in the manger"
...which calls attention to the fact that his refusal to go along with the majority detracts from other people's enjoyment, or is otherwise detrimental, and...
"He's playing devil's advocate"
...which accuses someone of arguing for a position they don't really endorse, simply for the sake of making an arguement.
I don't recognise the particular personality OP describes (would she be happy to accept a decision made by one other person? Would it make any difference if that decision affected just the two of them, or everyone else in a large group?). She might be any of...
[nonconformist, renegade, maverick, rebel, radical, individualist, heretic, dissenter, dissident, iconoclast, loner], etc., etc.
...depending on exactly what other characteristics she has, and on OP's value judgement of her.