So all ten Investigators are from other Arkham games? So.. Can someone give me details on Jim Culver and Rex Murphy?

What kind of details were you thinking off?

Here is some background from Arkham Wiki (background stories at the bottom of the pages)

Jim Culver

Rex Murphy

Here is the link to all investigators

Here is the same for the versions from Eldritch Horror - Bios are also on there but not all investigators are released yet.

I just wanted a general description of their personas and backstory stead of waiting for Dunwich I didn't think there be a wiki.