James Knight, MD (Dedicated Villain and Mandarin of Mayfair)
Super Bonus Answer: Tio Glendenning
At the end of Practice to Deceive, Gordon "was perfectly sure that he recognized the twinkling eyes of one (the ostlers carrying off the wounded Roly) as belonging to a dashing young peer named Horatio Glendenning." And he's a friend of August's in Jewelled Men series.
Obvious Bonus Answer: Trevellyan deVillars, because he knows everybody.
Name the people who do not know Trevellyan de Villars.
This is a trick question: EVERYONE knows Treve!