The official Tumblr account for Splatoon 2 provides a fair bit of information for the Octo Expansion DLC that is coming the game in Summer. The Japanese Twitter account has tweeted some extra information, shedding light on Pearl and Marina’s role in the new story mode. They can help you skip levels to progress the story faster but is is not known whether there is a limit to this or not.
Another tweet confirms that there will be no difference in gameplay between Inklings and Octolings in multiplayer and they can all wear the same gear.
Source: Splatoon Twitter account (Japan)