[@Rougespartan181] It's the spirit conscience of a beast from Zharysk operating as something similar to an A.I. inside of Kell's arm. She's conscious, so she does sleep. She can indeed move the arm, but Kell has mechanisms in place to prevent that from happening. Doesn't always stop her from giving Kell a pleasant slap to the face occasionally, however. She sees/hears/etc. things via Kell's eyes/ears/etc. and the gemstone on the arm. Momo can be transferred to another device if the gemstone she's harbored inside of is moved to an alternate home. 

As you can imagine. He removes the arm when it comes to bow chicka bow wow times.