Which expansion added playable Octolings to Splatoon 2?

The expansion that added playable Octolings to Splatoon 2 was Splatoon 2: Octo Expansion. It is a paid downloadable expansion centered on an amnesiac Octoling called Agent 8.

Released on June 13, 2018, the expansion takes place in the Deepsea Metro, an underground network filled with testing chambers. Agent 8 works with Cap'n Cuttlefish and receives help from Pearl and Marina while attempting to reach the surface. The campaign contains up to 80 challenge stages, with varied objectives that go beyond the standard structure of Splatoon 2's main story mode.

After completing the required story progress, players unlock the ability to choose Octolings as playable characters in multiplayer. Before this expansion, Octolings appeared primarily as hostile non-player enemies. Hero Mode and Octo Valley are associated with Splatoon 2's base single-player content, while Return of the Mammalians is the campaign for Splatoon 3, so none of those names identifies the relevant expansion.