Sriracha!Asriel is a glitch.
He's an unfinished sprite that was mistakenly added, but can only show up inside of Sriracha!Asgore's castle near the end of the Pacifist Route. He is an character from Srirachatale.
He is missing his left foot and has black, blue and green pixels going in certain places on him. He has grey fur with dark grey markings on his face. He has small red, green, and yellow horns.
He does have a boss form, but it turns into a large glitched pixel within 5 minutes of use.
Toriel and Asgore
Sriracha!Toriel and Sriracha!Asgore as no knowledge of his existence, they assume Sriracha!Chara was their only child, well, adopted one.
Sriracha!Sans seems to be the cause of his existence, having the ability to sift through files when the "game" is closed.