The playable Octoling in Splatoon 2’s Octo Expansion is Agent 8.
Agent 8 is an amnesiac Octoling who awakens in the Deepsea Metro, an underground subway system filled with testing stations and dangerous challenges. The character is initially identified by the Telephone as “Applicant 10,008,” but Cap’n Cuttlefish gives the Octoling the shorter field name Agent 8.
The Octo Expansion campaign follows Agent 8’s effort to escape the Metro and reach the surface. Players complete up to 80 challenges, collect four objects known as “thangs,” and receive guidance from Cap’n Cuttlefish, Pearl, and Marina. Completing the expansion also unlocks the ability to use Octoling avatars in Splatoon 2’s multiplayer modes.
Agent 8 is distinct from Agent 3 and Agent 4, who are associated with other parts of the Splatoon series. Marina is not the playable Octoling; she is one half of the musical duo Off the Hook and helps Agent 8 remotely during the adventure. Agent 8 later returns as the protagonist of Splatoon 3’s Side Order expansion.