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. 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. To activate it, the sword must be quickly unsheathed from its scabbard. This is however not the case if one is wielding the Spear. 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 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 and to cleave a turned off Kogetsu. Although it is unknown if a normal Senkū slash would be equally powerful, it is very likely it could replicate those feats, as Tachikawa managed to bisect two Rabbits in spite of their durable armor, suggesting its power can surpass 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.[BBF Q167] Senkū's only weakness is the necessity to sheathe Kogetsu, which makes it impractical at close range to all but the most skilled users. Furthermore, opponents who are familiar with the Trigger will be able to anticipate the attack and either prepare to dodge or make a pre-emptive strike in order to interrupt it.

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 unsheathed right before using Senkū is probably a reference to, where one draws their sword from the sheath only before striking.
 * The first time Senkū was used, Tachikawa activated it without sheathing Kogetsu first. So far, this is the only unambiguous case of this happening, implying it may have been changed later on.
 * Kyōsuke Karasuma, Shūji Miwa and Kotarō Tomoe are the only three official agents with Kogetsu but not Senkū in their Trigger set.