Frisk and Asriel's friendship is a low-key version of this in general. Even though they have only known each other for a short time, they already act like siblings. They frequently and casually do nice things for each other, eg: Frisk giving Asriel sunglasses or Asriel buying Frisk and Monster Kid nice cream. Considering Asriel's general moody and sarcastic demeanor and the fact that in the game Frisk forgiving Asriel is optional, these small gestures of friendship are very sweet.
The "Childhood" arc, while mostly a Tear Jerker, ultimately ends on an uplifting note: Asriel's therapist assures him that his thoughts and feelings are common and normal, and that he's a stronger person than he thinks he is, as evidenced by the fact that he ultimately avoided a war and saved everyone. His therapist also encourages him to find comfort in his friends and family. The arc ends with Frisk and Toriel hugging Asriel together.
In the "Unpleasant" arc, there's a bonding scene between Asgore and Asriel, where Asgore gives advice to Asriel on how to deal with guilt and grief.
Also in the "Unpleasant" arc, Frisk joining Asriel on his walk to comfort him after he argues with Toriel.
The "Bad" arc, like the "Childhood" arc, does not have a bad end: Toriel apologises to Asriel and assures him that she loves him no matter what, and it ends with Asgore, Toriel and Frisk all hugging Asriel together.
Toriel's Character Development in the "Unpleasant" and "Bad" arcs is basically a Decon-Recon Switch of her character in the game. In that in her anger, she has been hurting her children while also showing the hypocrisy in her pettiness and behavior toward Asgore, even though Asriel has done worse. However, it is reconstructed as she realizes what she has been doing and helps grow past that.
Also from the "Bad" arc, Toriel swallowing her pride and going to Asgore when she realizes that her viewpoint of the past might be a little skewed. Asgore, for his part, is more than willing to help, even though it means recalling his most painful memories - his children dying, his grief-fueled rage at humanity, and Toriel abandoning him in disgust at his actions.
Especially worth mention is how, once they finish recounting that painful memory, Toriel and Asgore find themselves bringing up happier memories of Asgore's past of impulsive decisions, even sharing a laugh together. This is especially significant for Toriel, considering how she'd been sniping at Asgore up till now.
Finally, as Toriel narrates her own Jerkass Realization, Asgore cuts her off and insists that he doesn't want her to blame herself - he just wants to help her, and Asriel, as much as he can.
Asgore: More than anything in the world, I want to see (Asriel) happy. I know you want that, too. So you and me, we're gonna make that happen, no matter what. Step by step, but we will. He deserves at least that much from a couple of sad old monsters...right?
