Ooh relationship talks reminds me.

If you guys wanted Marvin to have worked for you, or for your settlement/faction, go ahead. As I stated, he took on any mission he was presented with no matter which faction (though I do guess that Marvin left the Wasteland around thirty years before the start of Fallout 3's events). 

[s]Heck, even when he was with the Railroad, he took on some Instiute jobs under their noses.[/s]