You were sent on a mission to slay a Demon. You did manage to do so but before the demon died completely it turned you into a child. Hopefully there were more Slayers that brought you to the Hashiras and Master. They decided to make the Snake Hashira take care of you while you were in that state since he was the only one who could do so

Right now you were on Obanai's estate. He looked at you for a while before speaking

“I can't believe I'm the one taking care of you...”