Asriel is the protagonist of Deltacreep.

By observing the recent art @SweetKairamell has made, we can infer that Lancer will NOT be the protagonist anytime soon. In fact, it's safe to say Lancer will be with Kris and the others, or the "Creepers" as they are called now. And considering Asriel first saw Creep Kris outside the window in the story we have been given, it's safe to assume that we will control Asriel throughout the visual novel.

However, there are only two other people that could possibly fit into Asriel's shoes if Asriel is not the protagonist either: Seam or the Vessel. Yup, that's right, the Vessel. Considering that Gaster would want to end this anomaly, and possibly reset the timeline, the opportunity for the Vessel to appear would work great. Considering Vessels have human SOULs, one would be able to easily close the fountain or have enough DETERMINATION to RESET the timeline. Seam's inclusion as a protagonist is obvious, considering that Seam appears to be unaffected by this curse entirely. I'm taking Noelle and Berdly out of the picture since this is mainly around Chapter 1, not Chapter 2, since this is just the first Dark World.