How would you write a compelling character known for their kindness

I think the simple answer would be "Add another quirk or trait" like how Makoto Kino and Ami from Sailor Moon have a lot of interesting quirks going for them despite being such sweethearts. Or do have moments where they lash out or get angry.

Another one would be "Yeah, they are super nice, but that's because that's what they can offer to have worth, they have a lot of baggage and need to learn to be selfish as well." I would NEVER be able to pull this third one off, because it's just too complex, but I've seen this and I loved it.

I think the simplest way to do the third is just simply the baggage. I absotely love characters like Hana Nono from Hugtto Precure who are balls of Sunshine with a lot of sadness as well.