I've been sitting on this one for a while because I'll be the first to admit it's based on ever-so-slightly flimsy evidence.
We should all remember at the end of chapter 700 we got a look DD's three biggest generals, the slimy-invasive-guy who had the club's throne was shown, we'd already been informed that DD had been saving the heart throne for Law, and we were shown Diamante who now looks to be the first of them to be getting involved with Lucy's charge on the Colloseum, seated on the diamond throne. The first two's faces were revealed, however, the spade throne's occupant remained hidden, all we were shown was that he was tall, at least as tall as the other two who were equal in height to Doflamingo who is over 3 metres, and that he wore large, bulky looking spiked armour. And so, I've formed this hypothesis about his identity.
Doflamingo's spade is a marine soldier. Not only that, he's one we've already been introduced to.
I'm not very adept at playing about with picture's and stuff, so I'm just going to provide two links below:
In the second link, there is a marine who enters to report to semgoku about the 500 prisoners captured by Kizaru. He only appears here in two panels, I'm not very sure if he reappears later in Marineford, but I'll give it a skim tonight and include any other appearances that I find.
As I said, it's hardly a solid theory, however we already know that Doflamingo has had informants and subordinates infiltrate the Navy and use them to benifit his operations, Vergo masking the disappearances of the children on Punk Hazard for example, and it seems possible that the marine with the shoulder plates in the second link could have been working directly under the Fleet Admiral, little more than an errand boy despite his apparent rank of at least an officer. Doflamingo could have been using him to keep himself informed about both Sengoku and Akainu's movements for years! Yes, my theory is basically that DD has previous and this guy's got similar fashion sense, but I'm not denying that this is a shot in the dark. I'll provide any more info I can get on him as soon as I can.