Kogetsu/Senkū

The Senkū is an optional trigger designed for the the 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 causes a luminiscent slash to be launched from the Kogetsu's tip.

Abilities
The Senkū is usable in conjunction with the Kogetsu and by combining the two. To activate it, Kogetsu must be quickly unsheathed, and then the Kogetsu's blade can be temporarily extended and lengthened for farther reach. It can also be used to bend the tip of the blade as a way of temporarily increasing its speed and power as well. Experienced users of both Kogetsu and Senkū can take advantage of both's abilities to perform fast, mid-range slashing attacks.

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.