Grumpy sunshine pairs a closed-off, irritable character with a warm, open one. The sunny partner thaws the grouch, the grouch anchors the sunshine, and the romance grows from the contrast rather than from conflict.
The name is a reader's shorthand, two words that sum up a dynamic. The grumpy half is rarely cruel. He is tired, hurt, or simply someone who loves silence and hasn't been given a reason to break it. The sunny half is rarely naive; kindness is a decision she keeps making.
You spot the trope in the gestures. He says little and does a lot. She talks freely and notices exactly when he's listening. Care shows up as action: coffee left on the counter, a path cleared without comment, a coat put over shoulders without asking.
In “Snowbound” you are the mountain rescuer stuck for three days in a snowed-in hut with a loud, sarcastic snowboarder who won't stop talking. In “Inheritance of the Wind” the foreman who has guarded a ranch his whole life wants nothing to do with a gallery owner from Hamburg. And in “Stormstill” the lighthouse keeper Soren loves his silence until you break it.
At Everglow you do not just read it. You are the protagonist, and every chapter ends with your move.