The ending song suggest that Kris is struggling. "When the light is running low, and the shadows start to grow, and the places that you know feel like fantasy. There's a light inside your soul that's still shining in the cold, with the truth. The promise in our hearts. Don't forget: I'm with you in the dark." It's pretty clear what this is. If the "light" represents Kris's positivity, then it's being overtaken by brooding emotions. Parents are split up and mom hates dad, brother who was Kris's closest friend is now gone, he is treated like an outcast by a majority of the class and will never be the same as them physically. The storage closet literally feels like fantasy to Kris even though he's gone to that school for presumably a long time. Kris's hope is still there, but it's in the cold. He needs someone to reach out and rekindle that hope. It could be that Asriel literally is with Kris in the dark somehow. Let's not forget that these characters did have magic in Undertale. If we assume that they still can use it here, then it's possible that Asriel is attempting to project fun adventures with friends into Kris's mind, so that Kris won't feel so alone.
