Seeker

The Seeker is a sub weapon in Splatoon.

Splatoon
Upon deployment, Seekers chase after the closest enemy to them, leaving a trail of ink in their wake. Upon reaching the targeted enemy, they explode in a burst of ink. The Seeker's AI is not particularly intelligent, and should be aimed directly at the opponent if possible. The Seeker will also explode if it hits a wall. According to Judd, diving in one's own Ink will cause a Seeker to switch targets.

In Octo Valley they can be unlocked and upgraded to have a wider trail and bigger blast radius than the online Seekers.

Octo Valley
Seekers are one of three usable sub weapons in Octo Valley, unlocked with Power Eggs and selected via once available. They cannot be used in amiibo challenges, making Splat Bombs the only usable Sub Weapon for the Hero Roller and Hero Charger.

Use

 * Use it to ink long paths. Not only does it keep you at a safe distance, you can also focus on other things while the Seeker does its job.
 * This can be used to quickly ink your team a path to a Splat Zone at the beginning of Ranked Battles, giving your team an edge in capturing the zone.
 * In Octo Valley, use the seeker to take out far-away enemies and to ink long paths. It is especially useful in levels which require sneaking - it distracts Octolings and will ink a path for you to hide in.
 * Holding lets you select a target for your Seeker to chase. You can only target foes who aren't hiding in their ink, though, regardless of whether their position is tracked by your team or not.
 * This can be used to survey your surroundings, especially in Ranked Battle.
 * When moving on the ground, seekers leave an ink rippling effect nearly identical to that of an Inkling swimming through ink. Therefore, if a player swims through the trail of ink directly behind a seeker, they will become functionally invisible.
 * This can be used in many different ways. A player can follow directly behind the seeker all the way to the explosion of ink, or stop swimming at some point along the trail, allowing for some crafty surprise splats.
 * If left uninterrupted, the Seeker can travel for a maximum of 5 seconds before it automatically explodes.

Ink Management In-depth
With an extremely high ink consumption of 80%, some weapons are left dry after deploying a Seeker, and cannot be used to throw out two (It comes close, however, with only Ink Saver (Sub) stacked to max effect having the Seeker consume 52%, so replacing one main of Ink Saver (Sub) with a  will drop the consumption below 50% allowing for two Seekers). To fully capitalize on the distraction and mobility provided by the Seeker, a combination of the Ink Saver (Main) and Ink Saver (Sub) abilities is needed. The charts below show the combinations needed to achieve particular shot counts after using a Seeker. Alternatively, a combination of Ink Recovery Up and Ink Saver (Sub) is useful if the player just wants to throw more Seekers, as opposed to throwing a Seeker and then using the main weapon. After deploying a Seeker, there is a 1.17 second delay before ink can be refilled again.

Trivia

 * There are no Chargers or Splatlings with the Seeker as their Sub Weapon.
 * Seekers appear to be tiny hovercrafts, which matches their behavior of skirting over dry land and enemy ink.
 * Seekers would not return in Splatoon 2, with its two most notable traits instead being separated into new sub weapons; the sliding and ink-trail behavior would be turned into the Curling Bomb, while the homing behavior would be given to the Autobomb.