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 the player at a safe distance, they can also focus on other things while the Seeker does its job.
 * This can be used to quickly ink the team a path to a Splat Zone at the beginning of Ranked Battles, giving the 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 the player to hide in.
 * Holding lets the player select a target for the Seeker to chase. Only foes who are not hiding in their ink can be targeted, though, regardless of whether their position is tracked by the team or not.
 * This can be used to survey 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 surprise splats.
 * If left uninterrupted, the Seeker can travel for a maximum of five seconds before it automatically explodes.

Quotes
"Seekers run along the ground, spreading a narrow linear line of ink behind them - swim through this ink trail to quickly get around a stage. In their default mode, Seekers continue moving in a straight line until they hit an object, at which point they explode. But Seekers can also lock on to opponents. Hold down the sub weapon button to lock on to visible opponents - a reticle appears over your target. Release the sub weapon button to send the Seeker toward your target. While Seekers will attempt to chase down your target, they're incapable of making sharp turns or traversing steep inclines. But they can zoom right up most ramps, perfect for surprising opponents on the other side."

- Prima Guide

Ink Management In-depth
With an extremely high ink consumption of 80%, some weapons are left dry after deploying a Seeker, and Ink Saver (Sub) 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 Last-Ditch Effort will drop the consumption below 50% allowing for two Seekers. To fully capitalize on the distraction and mobility provided by a 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 do not return in Splatoon 2, with its two most notable traits instead being separated into new sub weapons; the sliding and ink-trail behavior has turned into the Curling Bomb, while the homing behavior has been given to the Autobomb.
 * Despite this, Seekers were planned to appear in Splatoon 2 and were to be usable in Octo Canyon.
 * The Seeker Rush is the only Bomb Rush variant to be added post release.