lindermans `friend.`

[phantom222]

linderman told nathan that he had a friend who he tried to save the world with, and that person abandoned him. i think it was nathans father.

[chrisxs5]

When Nathan was going to prosecute him, they sounded like the dad and Lind were still friends. I am thinking it was someone else with powers, maybe Devoux.

[Randy G.]

They're being purposely vague about it at this point.
I'm sure we'll get to see in flashbacks, that Ma & Pa Petrelli, Linderman, and Deveaux, were all key players in the events leading up to Today's Heroes. I also think we'll see the origins of Claude, Thompson, & Mr. Nakamura.

This is honestly, the best show I've ever been caught up in. :p