Reencountering the Ruined Knights as overworld NPCs if you spared them all. They're out enjoying a picnic together, their friendship is stronger than ever, and they now count you as one of their friends.
You can find the AnnoyingDog sleeping in the City of Old, and Chara notices its paw is cramped (a reference to Toby's real-life carpal tunnel). Chara hopes that the dog's paw heals so it can get back to "dog things," but also says that the dog shouldn't wear itself thin.
Chara: I'm sure it's appreciated by a lot of people, who wouldn't want to see it hurt.
Asgore picking Chara up to give them a bear hug. It gets even sweeter when you notice that the first time (when they make it to Asgore's house), Chara is the one who initiates it by holding their arms out for a hug.
Asgore's speech if you succeed in sparing him, with him gaining new faith that you will be all right and saying that everyone in Ruined Home will remember you. Topped off by what he says after giving Chara a farewell hug:
Asgore:Stay determined, Chara. No matter the distance between us... We will always be family.
Koffin-K comes across as this in general. Despite presenting himself as an antagonist, he's not a bad guy by any means necessary and is really just a mischievous troublemaker who's completely incapable of actually causing harm. Even Crossbones' clashes with the latter really just seems to consist of him humoring the guy.
On that note, despite insulting them (and badly at that), Koffin-K cares for his henchmen, Harry and Larry and his reasoning for having them both secretly do his laundry without them telling each other is his way of saying that he holds them both in equal esteem.
Koffin-K: IMBECILES! You've shown great passion, but you MUST work on your teamwork! Did you really think I'd favor one sidekick after another? I picked you both as a DUO. That means you are both equally useful, as you are equally inept.
Harry: W-wow, you...
Larry: You... really mean dat, boss?
Koffin-K: Of course I do! You're both IDIOTS! But you're idiots... TOGETHER!
After Chara has a memory flashback while standing atop the cliff, Koffin-K takes notice of how upset they were. While he initially passes it off as him as him only being concerned by their feelings interfering with his misdeeds, he backpedals and insists they talk to him if anything is wrong.
If you backtrack to Muffet's shop, she'll at first be unhappy, even if you try to apologise to her. But as you talk more, she'll eventually allow you to bake something with her, and opens up about why she's so sad and how she really wants to reunite the spider colonies. She soon warms up to you, and will be in a better mood every time you visit her from then on.
Crossbones willingly revealing his identity as Sans to Chara and welcoming them to his home at the end of the Starlight Isles on the Compassion Route. Although it was blatantly obvious to us as the players and it's played off as a joke, in-universe almost no one knows this, making this a clear indication that Sans that considers Chara a friend and trusts them.
Crossbones: you know, whatever happens... if nothing works out?
(Removes mask)
Sans: you always got a home here, Chara.
The conversation Sans and Papyrus have together while playing a fighting game during the hangout at the former's "Punishment Shack". It's a good reminder that, despite how different they are, these two are still brothers and they love each other.
Sans: playing against you reminded me of something. you're pretty cool.
Papyrus: Well, brother, I think you're cool too.
Sans: aw shucks. i'm not the one who makes all those cool gadgets.
