|Classification||Optional (Attacker only)|
|-||Increased slashing range|
|-||More powerful at the tip|
Senkū (旋空? lit. Rotation Void) is an Optional Trigger exclusive to Kogetsu, including its Spear variant. An attack carried out using this Trigger takes the name of Senkū Kogetsu (旋空弧月? lit. Rotation Void Crescent Moon).
Senkū's normal appearance is similar to that of a shirasaya (a katana's sheath). Since the spear form doesn't have a shirasaya, it is possible that it merges with Kogetsu's hilt to form the shaft of the Spear. When in use it lengthens Kogetsu's blade, which, due to the speed of the movement, often appears only as a luminescent ribbon of light originating from the blade.
The origins of this Trigger are unknown.
Senkū allows a the user to temporarily extend the reach of their Kogetsu by consuming trion. Naturally, the faster the movement, the more effective the attack will be. Senkū's main advantage is that it allows Attackers to fight at close-to-mid-range, albeit only momentarily.
Senkū's range is inversely proportional to its duration. The majority of Attackers activate it for 1 second, resulting in a range of 15 meters. Skilled users can perform multiple slashes during the same use: Kei Tachikawa and Tatsuhito Ikoma are able to carry out at least two attacks, Masafumi Shinoda up to four. With his Spear, Yōsuke Yoneya was able to strike five times in a row.
The lengthened blade is particularly dangerous at the tip. Compared to Kogetsu, which can crack but not break Scorpion with one swing, Ikoma's special Senkū Kogetsu was able to cut through two Scorpions and injure the user, as well as to cleave a turned off Kogetsu. Tachikawa managed to bisect two Rabbits with just one Senkū slash  in spite of their durable armor, suggesting the power of this Trigger surpasses even Ibis. Shinoda demonstrated an application of this Trigger similar to Mantis, lengthening the blade for piercing attacks for pinpoint accuracy. He also displayed the capacity to bend these attacks in order to avoid obstacles.
Senkū can and must be equipped on the same side of the slots as Kogetsu in one's Trigger set, hence it can can be activated even if two Triggers are already in use. Kogetsu users - excluding those wielding the Spear variant, as it lacks a sheath - are consistently shown drawing Kogetsu from its scabbard immediately before activating this Trigger and even sheathing and unsheathing it between uses. This action has proven to be a telling sign that the user is attempting the attack, alerting opponents and allowing them to counter by preventing the blade from being drawn. However, it remains unconfirmed whether this is a requirement.
|Double Senkū Kogetsu (ダブル旋空弧月 Daburu Senkū Kogetsu?)||The user activates Senkū through both the main and the sub while wielding two Kogetsu. This technique was very likely used by Kei Tachikawa to effortlessly cut down an Ilgar, whose already durable armor was enhanced as it was ready to self-destruct.|
|Ikoma Senkū (生駒旋空 Ikoma Senkū?)||By activating Senkū for a mere 0.2 seconds, the user can extend the reach of their Kogetsu up to 40 meters. This complex technique, which requires the user to swing their sword extremely quickly while synchronizing the slash with the activation of Senkū, is named after the only person in Border capable of using it, Tatsuhito Ikoma.|
See also: Senkū Users
There are 26 known users of this Trigger among official agents, including Yōsuke Yoneya, who activates it through his Spear. Masafumi Shinoda also has Senkū in his Trigger set. Tatsuhito Ikoma is hailed as the best Senkū user in all of Border. Shinoda and Kei Tachikawa are also worthy of note.
- The way Kogetsu is often unsheathed right before using Senkū is probably a reference to Battōjutsu, where one draws their sword from the sheath only moments before striking.
- The English translation of Chapter 154 heavily implied Senkū can be activated only while drawing Kogetsu; however, the original text makes no mention of it.
- When Senkū made its debut in Chapter 26, it seemed to project remote slashes instead of lengthening Kogetsu's blade.
- Kyōsuke Karasuma, Shūji Miwa and Kotarō Tomoe are the only three official agents with Kogetsu but not Senkū in their Trigger set.
|Basic Equipment||Bail Out • Radar • Trion Body|
|Attacker||Kogetsu (Kogetsu: Spear) • Raygust (Shield Mode) • Scorpion • Sōgetsu (Axe)|
|Gunner||Asteroid • Cobra • Gimlet • Hornet • Hound • Meteor • Salamander • Tomahawk • Viper|
|Firearms||Assault Rifle • Grenade Launcher • Handgun • Minigun • Shotgun|
|Sniper||Eaglet • Ibis • Lightning|
|Defense||Escudo • Shield (Affixed Mode/Full Guard)|
|Optional||Bagworm • Bagworm Tag • Chameleon • Connector • Dummy Beacon • Enhanced Radar • Full Arms • Geist • Gen'yō • Grasshopper • Idaten • Lead Bullet • Makō • Senkū • Silencer • Spider • Starmaker • Teleporter • Thruster • Timer|
|Prototypes||Connector • Dummy Beacon • Full Arms • Geist • Idaten • Makō • Teleporter • Timer|
|Black||Fūjin • Tsukihiko Amō's Black Trigger • Yūma Kuga's Black Trigger|