Lydia has friends who understand her. Considering how lonely she was in the film, it's pretty heartwarming to see her finally accepted by some of her peers.
Lydia and Betelgeuse's reunion is this in spades. He's exhausted and has almost destroyed himself with the effort to get to her; she was nearly killed. But their mutual relief and happiness at being together again is powerful. And then, despite his own condition, he takes her and all of her new friends to the place she most wants to be - home.
Their entire relationship could count for this, really, considering how far they've come in just two years. Lydia went from hating and completely distrusting Betelgeuse to calling him her best friend. He went from trying to use her for his own ends to selflessly defending her from any threat and valuing her above anything else in the world. "My girl," indeed.
In the flashback chapters, we learn what kind of person Betelgeuse was in life. As it turns out, he was a pretty good guy for the most part, and Katelin - his best friend's young daughter - loved him and never stopped believing in him. This sets the stage for his later attachment to her descendant Lydia.
Jack and Maddie Fenton's unconditional love for their children and acceptance of Danny's half-ghost status is nothing short of moving.
Adam and Barbara still don't like Betelgeuse (and it's hard to blame them), but they recognize that he really does care for Lydia and is good for her, and they appreciate that he's looking after her while they can't.
His heroism in the celestial battle to claim the Gem of Osiris earns Betelgeuse the right to move on to his eternal reward, free of suffering and sorrow. He politely refuses... because Lydia needs him more. The "higher-ups" who make the offer praise his selflessness and instead reward him with desperately-needed healing. May also count as a Tear Jerker.
The final chapter has lots of this. Betelgeuse is okay and he's back with his Lydia, who has been essentially adopted by the Fentons. She's able to visit the Maitlands regularly and text them daily, and meanwhile she's become really close friends with Danny, Sam, Tucker, and Jazz. Danny and Sam are still dodging the relationship question, but it's a foregone conclusion in everyone's mind.
The origin of Betelgeuse's apparent love of black and white stripes, back when he was alive. A crudely made mantle made by the daughter of his best friend who, at five, decided he needed someone to make clothes for him since he didn't have anyone else to do it, all while he himself was a village pariah. He wore the durned thing for years, even after she was able (and willing) to make him an improved replacement.
Say It Again
Whenever Betelgeuse notices Lydia shivering as the weather gets colder, he manipulates the air around her to keep her warm. He denies it, of course, but he doesn't want his girl getting sick.
Delia tries to bond with Lydia by carving jack-o-lanterns, and is impressed by Lydia's artistry. They have a nice conversation and Delia even promises to run interference with Charles so that Lydia can stay out late on Halloween. Considering how difficult their relationship has been up to this point, it's very sweet to see them both making an effort to improve things.