He can transform to Purifacation, a baby blue armor around him and powers of every Sans from the Multiverse.
Don't let his powers afraid you, he's still the lovable Blueberry from Underswap...Only, he got the same treatment like Papyrus, he cut, well used to.
He was first shown being Error's captive in the anti void and Error's hostage when fighting Stretch in AU Rumble.
After that he goes to Underswap and there on his own he finds his father, the previous royal guard leader, Gaster. And his mother, the royal scientist, Ariel.
Blue is one of the 4 original members to join, recruited by Ink. Since then his spirits and high hopes have returned minus... The innocent part of him.
At the start of the patrol he has showed an interest in Edge ( Underfell! Sans ) and started to fall for him.
They were each others valentine gifts but both never moved on after that. During the time Crystal and Cole were born.
Blue and Edge took it upon themselves to be their caretakers while they were alive.
Even though him and Edge didn't work out. One thing did work out was the relationship between Error and Blue. Error was still trying to be good when joining the patrol and Blue decided to be the one to change him.
Over time spending at Empress Light's place the two developed more then a platonic relationship.
It was hinted the patrol knew that the two had more then a friendly bond when both spent a large amount of time together, and were the reasons on how they confessed.
Giving Error the simple idea of the sakura trees blooming pedals wooed Blue's heart.
Besides Error, Cole, and Crystal, he has 3 more kids. BV, is basically their future son, while Hex and Glitch are their 2 twins.
His appearance represents a black long sleeves shirt that matches his pants, with a gray jacket to overlay it, with a dark blue hat, gloves, shoes, and scarf.
Don't think of him as the Underswap Sans you all know, cause he has the power to destroy everyone if his personality was that way.