Hadar is the relationship or romantic pairing of Halley and Radar. Throughout the show there are possible hints that they could be a couple:
Bend it like Radar: Halley makes Radar soup and gives him flowers but it turns out that he's faking his illness. And during the soccer game when Radar gets hurt from the ball, Halley asks if he's OK and Radar feels great!
Billy Starrider: Halley and Radar play with their dolls together in the beginning of the episode and Halley also gives Radar a ride on the flying surfboard.