So Ash is *Spoiler* ? theory E10
Voq.
Last episode with Voq; The Butcher's Knife Cares Not for the Lamb's Cry E04
First episode with Ash; Choose Your Pain E05
Memory-Alphas http://memory-alpha.wikia.com/wiki/Voq
Says Shazad Latif was cast to play Voq charactor but instead cast to play Ash. The actor who played as Voq is not listed.
As Ash has a histoy of existence before Discovery. Voq mutated to look like Ash with a memory implant /part brain transplant. Original Ash most likely dead.