This one will be brief, because this one is so obvious.
Kindness is the center of goodness. It is also different from being nice.
Kindness gives what people need. It is helpful.
Nice is for show and makes you look good.
Being kind means telling the truth in a way that does not hurt.
Being nice requires little white lies to hide the truth, but does not allow the person to know what they need to know.
Let's use the example people use when trying to rationalize lying...
Your friend just got her hair cut and you think it makes her look like a dog.
The nice person tells a lie and says, "It looks great."
The kind person smiles and says, "Honestly, it is not my taste. But if you like it and it suits you, then ok."
Because really, half the criticisms in the world are about TASTE anyway. You may think it makes her look like a dog, but someone else may think it makes her beautiful. Or it makes her feel empowered, or some sort of thing like that. And, for that matter, the haircut may grow on you. But for pity's sake, don't lie. They may have been fishing for a compliment...but more likely they wanted an honest opinion.
The opposite of kindness is meanness, spitefulness. Kindness is inclusive. It brings people in rather than pushes them out. True kindness is honest. It is sharing the truth in a respectful way. And some kindness may not come out as nice. For example: Some kids think it isn't nice that their parents restrict and set limits on their lives. Yet it is often the thing kids need the most. Imagine a child who hates sitting in the car, squirmy every time you put him in the car seat. But, is it kinder to let the child climb about or strap him in? Is it kinder to put the child at risk or to restrain him?
Just a thought.
This came to mind when I was thinking of this topic. For fun..