I have two questions first how exactly do soul mates work as in is there only one all the other stuff two how long will Aliza live for will she have a normal human life span or a monster one or maybe somewhat in between?

Hokay. That first one is a… Pretty loaded question, with a very long explanation. So I’m gonna water it down quite a lot.

Soul mates are what equate to two halves of the same whole, two souls meant to be across time and space. Monsters feel the need to seek this other half out their entire lives, as monsters are one with their souls, but humans, most times at least (barring the most in tune with their own hearts), have no idea they are incomplete and go on unaware. It is a unique occurrence, with only one soul match. Sans and Frisk were such a pair, though they were mated for a very short time, only about two years, and thus, Sans was spared death when Frisk passed on, only taking great damage to his soul at the loss of her.

As for Aliza’s lifespan, it will be on the longer side of the monster spectrum. Skeleton monsters live a great deal of time; Gaster was alive well into his 1600’s before he fell, and his sons were hypothesized to be capable of living just as long, before the Hunger and loss damaged their souls. Sans will be lucky to live to see 400, with the state he is in, and Papyrus… Remains to be seen.

Aliza, being composed of a magical body and a human-like soul, is a highly magical being, and could live either a little short of her father’s original lifespan, perhaps 1000 to 1200 years, or even longer, to 2000+. It depends a great deal on her experiences in life, and who she mates her soul to, as soul mates’ lifespans are connected once they bond.