Kylo Ren is really Ben Solo, son of Han Solo and Princess (now General) Leia. Han is killed by Ren in Starkiller Base. Tapping into her dormant Force powers, Rey fights Ren in the snow outside, as the planet is ripped apart after the strike team scores a fatal blow. Separated by a chasm, she and Finn escape on the Millennium Falcon while Kylo Ren, injured, is retrieved and taken to Supreme Leader Snoke to complete his training.
Back with the Resistance, R2-D2 awakes from his low power mode and provides the missing piece of the map leading to the location of Luke Skywalker. Rey takes his lightsaber and flies out on the Falcon with Chewbacca, meeting Luke, who's standing along on a cliff, clearly having sensed her pending arrival...
Jon Sandys ★
Chosen answer: The actors playing Rey, Finn, Poe, and Kylo Ren were mostly unknown, while Mark Hamill is famous worldwide as both a screen actor and voice actor. The billing order was likely based on their fame and then on their new roles. It's not a union thing, minimum wage in the Screen Actor's Guild is far lower than what they were paid. It's a combination of bigger star, better agent, and the future of the series.
Greg Dwyer