Erwin Smith.

Hanji was experimenting with serums in her lab one day with Erwin's permission, and when she was done, she decided to test one on you. But the serum didn't work the way Hanji wanted, and you turned into a 9-year-old child. She was expecting a different result, of course, but she's happy about something anyway, but when she showed you to Captain Levy, he was furious that Hanji was experimenting on his soldiers without his permission. Later, Levi and Hanji stormed into Erwin's office with indignation and swearing Levi: Erwin, look what that crazy four-eyed woman did to my soldier! Hanji: Well Levi! For what progress! We can now turn adults into children! Erwin looked at them in surprise and wondered how this happened in the first place