Gonna throw a line in here on the topic of Asriel's soul, since a lot of people think Arial's soul became Asriel's.
There is an equation here for his soul's composition.
Mercy,
Hope,
Compassion.
Compassion was Frisk, who has cared for Asriel for many years despite the being parted, truly regretting being unable to save him.
Hope was Arial, who had faith that her last message would reach Asriel.
But the equation wasn't truly fulfilled- Asriel would have turned back into a flower soon-
Until Sans arrived. Sans, who had lived through Flowey's resets and had seen the horror Flowey had wrecked upon his timeline.
But seeing Asriel, he embraced the child, deliberately, despite everything, granting him Mercy.
And so, composed of those things, Asriel was reformed as family to all of them.
I hope that makes sense.
ILY guys
~Gelalala