The butterfly or the mermaid?
Grieving people find a hundred ways to look at a butterfly, which is tricky, really, cause it's not as though they stop flying long enough for you to really see. Except for when they're printed on greeting cards.
The common idea is that the butterfly represents the beauty of the release from pain in the physical body, flesh become spirit. Death is maturity, life is pain, wings are eternal and lovely: who woulda believed this could come outta that icky caterpillar…? And so on. The butterfly shows us how our loved one shuffled off of his mortal coil.
Disabled people use butterflies a lot because they are free, nimble, they take the air easily, and their beauty is truly transcendent: beyond all our earthly understanding.
The butterfly is change, metamorphosis, radical. Worm to angel. As much as the old shape hurt to live in, I’m really sure that butterfly in the chrysalis is numb (apparently it’s a blob of mush in there for a while, neither/nor) and then awakes and stretches into exquisite pain. Not something you do unless you have to.
The butterfly’s also a symbol of the grieving person and how they are transmuted, painfully, into a beautiful creature, released from the grey shield, no longer walled inside our own home. And yes, we widows blossom spectacularly, if you wait long enough, if we can just remember that there IS a way through.
But grieving people can't change form or environment as quickly as the butterfly, and we're expected to last much longer.
And if I was to have movement -- or even "a" movement -- I needed something — an image, an icon, an avatar — with competitive color, to adore, something to look up or outward to. So I used the mermaid, even though I had to change the ending of the story (and I don't really know the NEW ending, either).