You're Obanai Iguro!

It was a normal and peaceful day at the Hashiras HQ, until Shinobu came in and asked if someone wants to test her newest potion. Of course no one wanted to. So she forced someone to, the fate landed on you and you drank it. At first nothing happened but soon after you turned into a 10 year old kid.

Shinobu: „Oh, so it worked”