Kogetsu/Senkū

"Kogetsu Optional Trigger: Senkū. A momentary lenghtening of the blade's reach through a trion expenditure. The swinging blade is most deadly at the tip."

- Senkū's description

Senkū is an Optional Trigger exclusive to Kogetsu, including its Spear variant. An attack carried out using this Trigger takes the name of Senkū Kogetsu.

Appearance
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.

Overview
The origins of this Trigger are unknown.

Abilities
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.

Users
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.

Trivia

 * The way Kogetsu is often unsheathed right before using Senkū is probably a reference to, 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.
 * That is also one of the only two occasions in which Senkū was unambiguously used without unsheathing Kogetsu immediately before. The other instance, albeit not as clear, can be found in Chapter 157.
 * Kyōsuke Karasuma, Shūji Miwa and Kotarō Tomoe are the only three official agents with Kogetsu but not Senkū in their Trigger set.