I think that the answer depends on whether this is a, or the, main character in the story.
The main character or main plot-driving character is the protagonist. Someone who opposes their goals is an antagonist--and often there is a "the antagonist" vibe, where the primary character opposing the protagonist is "the" antagonist.
Now, it's not always this neat. Bill and Ted's Excellent Adventure, for example, arguably has two protagonists. In the 1993 The Fugitive, Tommy Lee Jones' character is arguably the antagonist, except toward the end he turns around and he's helpful. In The Great Gatsby, the main plot-driving character, Gatsby, is separate from the character that we identify with, Nick Carraway. And so on.
Nevertheless, I think that the designation of protagonist/antagonist depends on who the main plot-driving character is, whether the character you're trying to classify tends to champion or sabotage their goals, and how important that character is.