Their name is 因果报应.Sans posessed by Chara.Chara was fed up with you slaughtering everyone,so they fused their ghost with Sans soul,creating a new entinity named 因果报应 or simply Karma.Karma uses they/he pronous.They just want revenge for murdering everyone they know,but If you reset,they will forgive,thought No chance they will become seperated again.They are half melted skeleton in green jacket with yellow slippers.Right part of their body is melted.And they have a huge slash across their chest.