Any and all friendship moments between the two leads.
Likewise, Brago's Character Development and friendship with Sherry. He starts out as a selfish, borderline psychotic creep who only desires to be king for the power of it, but by the time the conflict with Zophise is done, he has became a noble person who cares about Sherry's feelings and problems, and the feelings of others, and desires to be a truly benevolant king.
In one chapter:
In one of their searches for a momodo, when deciding to return to the hotel after finding nothing. Brago tells Sherry to go back without him saying shes to weak to keep up, but Sherry's butler only points out that what Brago really wants is for her to rest.
There is even a flashback in that same chapter. During a training session 10 months before, Brago didn't want to put up with anyone interfering with their training and fully believed anyone who was in his way deserved to die. Brago would only be kept in line by Sherry's threat to kill herself and preventing Brago from becoming king without a partner. She even said he was no different from Zophise, thinking so low of humans and taking what he wants from them.
Brago's words to a boy:
Brago: Brat... crying won't change anything. Become strong! If you do that, you won't have to suffer painful memories because of those bastards.
Sherry is utterly surprised by this.
As well as their final battle with Zatch. She gives it her all, but Zatch's power overcomes Brago's. She starts to cry, but Brago gently takes her hand and tells her she did good and was glad to have her as a partner. Sherry hugs him in response as his book is burned.
Every single Heroic Sacrifice in the series is one of these. There are too many to count.
Zeon's reconciliation with Zatch, and also telling Dufort to stop saying his life is meaningless. In turn, Dufort realizing that he does indeed have something to live for.
In the epilogue, we see Zeon telling off the abusive old lady who raised Zatch right before the latter's coronation. He became a proper big brother after all!
Kiyo's Character Development was made clear to his dad after the battle with Baltoro; he came all his way to the castle to save him, and was willing to put himself in danger to save Kory/Sekoro's parents. It was a complete 180 to his original jerky persona, and his father couldn't be any more proud of that, or thankful of Zatch's help.
The story behind Shin and his late mother in episode 10, and how Kiyo and Zatch indirectly teach Shin to be the sort of man his mother was wanting him to be.
Kyle finally standing up for himself and demanding that Jill stop wasting his father's money. Zatch and Kiyo watch on as they see Kyle becoming just like Rein said he would. Rein tells Zatch that he'll be there to support him when Zatch is king and encourages him further.
Momon holding out solo while waiting for Kiyo's return, and Kiyo making it in time to congratulate Momon before he disappears. Sister Elle tells him how proud she has become, and Momon promises to dedicate himself to studying and becoming as smart as Kiyo. The ending shows he's keeping that promise.
The end of the Demon Mirror filler arc. "Nya, I'm going to be okay from now on."
Kanchome extending his hand of friendship to Goomu.
Goomu returns the favor by confronting Clear Note about letting Kanchome live in the momodo world, but since that wasn't the agreement between them Clear Note refuses. Goomu shows up to Zatch half alive and gives him Clear Notes location with his partner asking for Goomu to be spared. Zatch and Kiyo vow to save him also.
The grand finale shows Kanchome, Purio, and Goomu all happily heading off to school together while the latter is letting the two smaller demons ride on his shoulders.
In the final battle with Clear, everyone showing up and lending Zatch their powers, starting with Danny and continuing through almost every major demon friend Zatch and Kiyo made. And yes, it even includes Kolulu, who shows that she finally has a non-violent spell that ends up vital to our heroes' success.