The "Chemistry" Leg. This is the spark created by their differences. It’s what makes the banter pop and the tension simmer.
The "External" Leg. The characters must need each other to achieve a tangible goal (e.g., solving a mystery, winning a competition). This provides the "Why now?" of the romance. stool leg sex
This creates power imbalances. If one character provides all the emotional vulnerability while the other only provides utility, you have a recipe for a compelling "unrequited" or "learning to love" arc. Examples in Popular Tropes The "Chemistry" Leg