Nice is modifying behavior/ tone due to fear of confrontation or loss, or a socially conditioned response to fit in and repress what may be deemed ‘unruly’
Assertive is getting needs met with as much of a win-win situation as possible
or win-lose if others are bad eggs but never you lose-another wins
Kind is treating others how you want to be treated or even how you imagine they want to be treated, with as much Love, from a wellspring of self-Love and well being, and truth as you deem they can handle.
I strive to be assertive and kind.
The higher my sense of self and confidence the easier this is.