Petyr Baelish.
Låt oss höra det direkt från lejonets mun:
"To be sure," Lord Emmon said. "Ser Jaime, your lord father's faith in me was well placed, you shall see. I mean to be firm but fair with my new vassals. Blackwood and Bracken, Jason Mallister, Vance and Piper, they shall learn that they have a just overlord in Emmon Frey. My father as well, yes. He is the Lord of the Crossing, but I am the Lord of Riverrun. A son has a duty to obey his father, true, but a bannerman must obey his overlord."
Oh, gods be good. "You are not his overlord, ser. Read your parchment. You were granted Riverun with its lands and incomes, no more. Petyr Baelish is the Lord Paramount of the Trident. Riverrun will be subject to the rule of Harrenhal."
-- A Feast for Crows, Chapter 34 ("Jaime V")
Det ser ut som ditt Wikia-citat om Lord Walder Frey är typiskt Wikia-nonsens, eftersom jag inte kunde hitta några bevis för det i texten heller.