She's seen as a baby thanks to her illusion Semblance

Emerald Sustrai had a very impressive Semblance, all things considered. She could craft detailed hallucinations to trick her opponents with, and it was key to the plans her leader, Cinder, came up with. One of the key moments that led to the Fall of Beacon and Cinder absorbing the rest of the Fall Maiden powers was because she used her semblance to trick Pyrrha Nikos into killing the robot girl Penny on live television.

Her Semblance could only affect a single person at a time, but she had been training to make more people fall for her tricks at once. Said training advanced in a way she didn't expect it to. When it seemed like Cinder had perished during the Battle of Haven, the emotional response from Emerald caused her Semblance to go into overdrive, affecting numerous people in the area. When she woke up, she noticed her room in Salem's palace looked a lot more different, almost like a nursery. Her room was full of baby clothes, toys, and all kinds of infantile furniture. Even her bed had become a crib. Emerald looked at herself and saw she was dressed in a bright green onesie with what she could only assume was a diaper underneath.

Naturally, Emerald wanted to know what was going on. "Hello? Is anyone there? Hello?" She cried out until Cinder walked into the room.

"Don't worry, baby, Mommy's here." Cinder said in a light tone of voice, only confusing Emerald more.

"Mommy? Cinder, what are you talking about?" Emerald asked as Cinder approached her and lifted her out of the crib.

"Did you have a nice nap, Emmy?" Cinder just ignored Emerald's question with her response and started carrying her out of the room, no matter how much she struggled.

Looking around to see any kind of clue, Emerald saw a mirror nearby and saw the reflection of Cinder carrying her. The only difference was that in her place, what looked like an actual infant was being carried instead, as if that was how everyone else saw her.