Either because the love interest doesn't want their love to be seen or due to the MC being dense to their advances, the love interest steadily and stealthily displays their love for the MC (or just gives microscopic hints) without the MC finding out until very late in the game. It could almost be considered an unrequited love. Usually shows the love interest's POV at some point.

Based off of the books I've read so far and who's romance I find endearing.