Sans doesn't react to someone approaching until the exact moment that J is reaching to take the crystal away from him. He jerks it away from J's grasp reflexively, holding it closer to his chest, his eyes startled and wild as he repulses from the new arrival's presence. He shuffles his legs to take him further away from the man - wincing as it puts pressure on his wounded leg - but otherwise doesn't seem to notice anything besides J, the crystal, and their relative proximities.

"B-Back off," he sputters, his mouth feeling like it's running on auto-pilot. J doesn't understand what this is, or what happened. He needs to stay away.