He's nobody in particular. You would see this encounter at the near-end a neutral or pacifist route. Phase 1 - FOREVER PHANTOM HEAT *You see sans at the end of the corridor. *He cracks no puns as he approaches you. *Get ready. If you're on a No-Mercy route, you skip immediately to this phase. Phase 2 - FOREVER PHANTOMS OF NAZARETH *SANS GRABS YOU AND BREAKS YOUR RIBCAGE! *50% MHP REMOVED. *Broken Bones debuff: Your movement is cut in half. *...You feel hopeless(neutral run). *...You feel DETERMINED(pacifist run). KILL: You stumble out of the corridor and cough up some blood. You made it, although not without sacrifices. The survivors greet you with open arms and get help. NO MERCY KILL: You stumble out of the corridor, and cough up some blood. You made it, although not without sacrifices. However, as you look around, nobody is there to help you. You feel alone. SPARE: You knock him out and stumble out of the corridor. The survivors greet you with open arms and tend to your wounds.
Wanted to expand on the AU a little more so uhh yippee here ya go Papyron for the original sprites eyescreaming for Phase 1 theme (FOREVER PHANTOM HEAT) Nodel for Phase 2 theme (FOREVER PHANTOMS OF NAZARETH)