contrarius

Contrarius, contraria, contrarium is a first and second declension adjective built straight off contra. It takes a dative for the thing it is opposed to, "contrarius rei", or contra with the accusative when you want the opposition spelled out. Comparative contrarior is rare; the sense is usually absolute enough not to need degrees.

The phrases carry more weight than the adjective alone. "E contrario" means on the contrary, "in contrarium" to the opposite effect, and the neuter contrarium standing on its own is simply "the opposite". A trap for English readers: contrarius is not a temperamental judgement the way "contrary" is of a difficult child. It describes position and direction first, and only then disagreement.