He is one of Dexter Morgan's many victims.
He's identified via a blackboard in the background during a meeting about The Bay Harbor Butcher, with Detective Madison being the one covering his case.
Agent Frank Lundy unzips his body bag in the morgue to reveal a pile of bones, telling Debra Morgan, Detective Ramos and Angel Batista that he was a janitor for a school district and has no criminal record. Debra later confirms that Thatcher was a murderer, but it is never revealed who he killed, why he killed them, or how Dexter found out about it. Debra somehow discovered proof that he was a murderer.