After a journey through Syd's mind in "Chapter 12" of Legion, Lenny showed up at Division III to bring the series back to its surreal reality. How Lenny on Legion is back is anyone's guess since David's friend from Clockworks supposedly died after David switched bodies with Syd in Season 1. But, thanks to the Shadow King, Aubrey Plaza's character has been popping up all over the place — and now she appears to be alive again in the real world. But you can never assume anything is what it seems on Legion, so there are plenty of viable theories on how Lenny has returned.
In "Chapter 10," Lenny reminds David of all the times they had gotten high together. But David knows now that those memories weren't real; he hadn't known Lenny before he was admitted into Clockworks. Instead, Amahl Farouk had used the idea of Lenny inside of David's mind. When David wonders in "Chapter 10" if the Lenny he's seeing now is even partially the Lenny he knew in the psychiatric hospital, Oliver responds, "My associate is who she needs to be at this moment."
So perhaps the Lenny who appears in Division III in "Chapter 12" is also just being who she needs to be. But why? These six theories about Lenny on Legion might help solve that answer before "Chapter 13" arrives on May 1.