Silver is basically a mindless Stand version of Alphonse Elric, it's a powerful Guze King but has no will of its own. It was originally meant to be the avatar of Sairei no Hebi since it was powerful and is actually composed of ten separate Stands (each, say, as powerful as the original Silver Chariot) but Sairei no Hebi didn't like it because it was boring.
He's much more attracted to Yuuji - rather, Sairei no Hebi is GAR for Yuuji. He was awake when he saw Yuuji defeat Sabrac - Hebi already had a lot of respect for humans and a normal human beating one of the strongest Guze Kings like Sabrac really impressed him.
So he offered Yuuji to merge into one being - together they could rewrite the whole universe, Yuuji could be with Shana and Hebi could get his perfect world where humans and Tomogara live in harmony with one another (both goals weren't mutually exclusive btw).