Derplander is the stand-in for the Player Character. The change to Dragoon does two things: 1.) Represents the shift to Heavensward (which also means the 4.0 trailer will have him changing to Monk.), and 2.) So if he ever appears for more then a few seconds in the trailers showing off the Edgeparty he looks sufficiently different from Edgelander so we know who is doing what. While the 3.4 trailer barely shows him he is seen atleast twice. Once very briefly when Thancred goes to meet Edgeladin's attack head-on, and then when he is shown being Holmgang'd by Edgelander. The first shot we see him as normal but he isn't the focus of the shot, the second time he is the focus of the shot but is greatly shadow'd. This might be because even with the job change SE realizes its just kinda wierd, but it also might be when our character is similarly chained up we will be cast in shadows as well.

IF Derplander actually exists in-game he won't be the Warrior of Light, but would most likely be used as evidence for the Warriors of Darkness being from and alternate Eorzea. Maybe with a little humor of us and allies thinking he is Edgelander and trying to fight him.