Supernatural has an odd case of a love interest being derailed for reasons unrelated to their being a love interest. In season 4, Dean meets Anna Milton, a fallen angel whom he quickly takes a romantic attraction to. Originally, she was going to replace Castiel (who was going to be killed by Alistair) as Dean’s angel guide, however this hit a snag when Castiel became the show’s Ensemble Darkhorse, with both fans and critics responding more positively to him than any character other than the Winchesters themselves. Thus, Castiel was spared and his role in the series expanded. This had the unfortunate side effect of making Anna redundant, however, and she was given only a few more appearances before Jumping Off the Slippery Slope and deciding she needed to erase Dean and his brother Sam from existence.
TV Tropes, “Derailing Love Interests” (x