You were a Human just like Frisk fighting Betty Noire to stop this whole madness. Sans died, Papyrus and Gaster had to take Sans soul back to bring him back soon as possible.

Betty Noire: "Just give up~... you know deep down your fate is in MY hands."

She held your soul out and gave it to Akumu. Frisk looked at you not wanting you to do anything stupid.

Betty Noire: "That's right~... Just surrender and let ME KILL YOU!."

Akumu without realizing accidentally stabbed Betty Noire through her chest right into her Soul. Akumu caused your Soul and her Soul to swap bodies from an unexpected error in their attack. Betty Noire slammed Frisk to a wall using your body, seemingly according to her plan.

Frisk: "AGH-.."