The main character has a friend who isn't the best looking, but after some plot development the realization of what he/she really means to them becomes overwhelming at one moment. The moment when he/she suddenly becomes beautiful to them. Example: Rumpelstiltskin towards Belle when he catches her falling off of a ladder in Once Upon A Time.
Could also be a YMMY trope when a character on a show is not exactly the best looking, but they become beautiful to the viewer based on how the actor portrays the character or how much the character impacts the viewer. Example: Jeremy Brett in the Granada Sherlock Holmes Series or Matt Smith in Doctor Who