Well Eames, of course!!
Although it was never absolutely clear until Blind Spot, I knew I had heard reference to Eames being the senior partner very early on in the series. I finally found the episode today! It's in The Third Horseman.
Of course it's always hard to know if Goren is yanking someone's chain or if it's a truth..... so it really wasn't clear she was the senior partner. They were definitely playing Mr. Cutler, although Eames was not hiding her true feelings.
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