More datamining has revealed more about the Octolings! Previously, "Oct" and "OctBoy" were found in the game's executable file, near the PlayerType variable, but it wasn't entirely known if they were meant to be values for that variable. Now, however, they definitely are.
Additionally, "Oct" is now revealed to be "OctGirl", and "OctBoy" remains. The "RivalWeak" and "Rival" refer to enemy Octolings found in Single Player.
This image has also been found in the game's files:
On the right is the Octoling hair we're all familiar with, but on the left is what looks to be a possible hair model for "OctBoy", though if this is the case it appears to be a work-in-progress.
EDIT: This image was actually sent to the dataminers privately, so it's not guaranteed to be legitimate. However, they decided to inklude it in their post on Reddit, so it's possible it holds merit.
This last bit doesn't exactly mean much in the way of playable Octolings, but I felt it should be included anyway.
There is an Octoling icon present in the game which would have appeared if an Octoling spoke to the player during the campaign. It still has those signature goggles from Splatoon 1 though, so it's very possible the idea was scrapped early in development. My unreasonable side wants to say this is for an Octoling story episode they have planned somewhere down the line though @u@