Who is the guy in the trailer? Us or Arbert?

[Enkidoh]

Derplander is both, he both represents the player's character as a stand-in in cutscenes and propaganda artwork, and is his own independent character in the form of Arbert. Ferne confirmed this way back during the Beta test for ARR on the old forum that is now archived:

So yes, the Midlander male, Miqo’te female, Roegadyn male, Elezen female, and Lalafell female in the recent trailers are in fact the same ones that appeared in the 1.0 trailer! (Yes, the black mage is the Elezen female from the morbol party, not the snooty male from the Midlander’s party. The robe & hat conceal some of her more distinguishing features, and we all know there was no bust size adjustment feature back in 1.0).

But that still leaves one question—was the party an actual party, or were they merely symbols meant to represent you, the players? Well, you can breathe easy knowing they are both. They all ventured into Eorzea at the same time as you did. They all did similar quests, and lived through similar adventures. They exist as your characters exist (in a virtual world-sort of existence). The fact that you haven’t come across them is probably the same reason you haven’t come across my character, Amanda Hugginkyss (or maybe you have? /wink). They’re just somewhere else. Though, maybe you’ll stumble across their party in the future. Or maybe you won’t. How’s that for committing to an answer?

And that still works even with the revelations in SHB. Derplander, is you, and is his own person (in a metaphysical way). Commence mind screw.