Some female Octolings serve the Octarian army as elite soldiers, who can be encountered as enemies in single-player modes of Splatoon and Splatoon 2.
The Octo Expansion DLC of Splatoon 2 introduces playable Octolings who look significantly different from the Octarian soldier ones and could be unlocked for multiplayer upon beating the DLC. However, despite the player controlling an Octoling in the DLC, Octarian soldier Octolings still appear as enemies, albeit "sanitized" (brainwashed by the Kamabo Corporation). Octolings were also temporarily playable in multiplayer without the DLC for players who chose Team Octopus in the Squid vs. Octopus Splatfest.
Though this cannot happen in-game, lore-wise a Splatoon 2 Anniversary Famitsu interview reveals that only several female playable Octolings can become the aforementioned elite Octarian soldiers.
As a costume
Octoling appears as a Mii Fighter hat as part of Round 11 of Ultimate. The design is based on the enemy soldiers encountered in the single-player campaigns.