Warning! This article contains SPOILERS for Dexter: Original Sin season 1, episode 4 and Dexter season 2.A dark theory about Dexter: Original Sin could explain why Tanya Martin (Sarah Michelle Gellar) never appeared in the original Dexter series. The cast of Dexter: Original Sin features quite a few returning names from the original show, like Vince Masuka (Alex Shimizu), Angel Batista (James Martinez) and Maria LaGuerta (Christina Millian). There are only a handful of entirely new characters at Miami Metro, one of whom is Tanya Martin. Since she was never mentioned in the original series, the prequel show will have to find a way to remove Tanya sometime between when Dexter: Original Sin takes place and when the original show began.

https://screenrant.com/dexter-original-sin-tanya-martin-jimmy-powell-killer-theory/