Honestly, I've been chewing on this one for a bit because I think the most effective villains in romance aren't always the 'evil' ones. They're the ones who create ideological or situational conflict that tests the core values of the relationship. Think about rivals who aren't necessarily 'bad guys' but represent a legitimately different, tempting path for the protagonist. Like in historical romances, you'll often get the 'duty vs. love' conflict personified—the other suitor who offers political stability and societal approval versus the passionate but scandalous love interest. That's a villain in function, not necessarily in heart.
Then you've got the internal villainy, which is my favorite. A character's own pride, past trauma, or secrets acting as the main antagonist to their happiness. The 'villain' is literally a part of them they have to overcome to be with their person. That always hits harder for me than a cartoonishly evil ex showing up to cause trouble. It makes the drama feel earned.
But yeah, the classic external villain works by threatening the relationship's very foundation—spreading lies that erode trust, using power to physically separate the couple, or revealing a secret that fundamentally alters how they see each other. It's less about fighting the villain and more about watching the couple's bond either fracture or solidify under that pressure.