Walder Frey

Walder Frey is the so of Genna, and Emmon Frey making him a member of House Frey through his father, and a member of House Lannister through his mother. Walder Frey is the only son of Genna Lannister that was actually fathered by Emmon Frey and thus the only one of her children that is truly a member of House Frey.