Sengetsu Tsuki

Sengetsu Tsuki (月せんげつ, Tsuki Sengetsu) is a Shinigami from the Tsuki family and the twin brother of Maryu Tsuki. Unlike his brother, he isn't married. Despite Maryu and Sengetsu being twin brothers, they look nothing alike, Maryu's hair being spiky, messy and red, and Sengetsu's long, straight and black. However, they are commented to have the same eyes.

Personality
Sengetsu's personality has always been much colder outwardly than his brother's However, in his heart he bears strong feelings for the members of his family, especially his brother. His relationship with his Zanpakutō, Kikitsune, is difficult from time to time, because Kikitsune is fond to test Sengetsu in ways he has hard time to understand. However, he has stated he is close with Kikitsune. Sengetsu smiles rarely and is visibly angry even more rarely.

History
Sengetsu has gained fame because he has spent most of his youth hunting down his brother who had turned in to the calls of his bloodthirsty Zanpakutō, Kyūbimaru. During this hunt, he learned to know his own Zanpakutō, Kikitsune and eventually learned both its Shikai and Bankai. He used the latter for the first time against his brother, managing to defeat him after a final clash with the Ōken-state of his Tenga Kikitsune, thus bringing him back to the light side of the law. After Maryu's turn of mind, Sengetsu was forced to witness his brother developing an inner hollow, helping to push it back when it first tried to emerge.

Powers and abilities
Great Spiritual Power: Sengetsu is commented to have a great spiritual power, even though he himself admits it is not as strong as his brother's. The color of Sengetsu's reiatsu is normallu blue; however, it changes to white when Sengetsu enters Bankai.

Master Swordsmanship Specialist: In his fighting, Sengetsu relies on his zanpakuto more than anything else, being exceptionally talented in both the traditional form of Zanjutsu (like Kendō) and more unexpected forms of fighting, as his Shikai is not in the form of a reqular sword. However, he doesn't use similarily creative tricks as Maryu, whose Zanpakutō is far more out of normal.

Hand-to-hand combatant: It is known Sengetsu can do atleast basic Hakuda, but he hasn't shown so much skill in this area.

Kidō Expert: Unlike his brother, who never learned to use Kidō well, Sengetsu has learned Kidō and can use it effectively. He often utilizes Bakudō like #61: Rikujōkōrō to supplement the offensive powers of his Zanpakutō.

Exceptional Speed: One of the most renown abilities of Sengetsu is his high speed, augmented further by his Bankai. He is notably faster than his brother, and can move as fast as his Zankō Kyūbimaru's blades can extend while in Bankai state.

Zanpakutō
Kikitsune (鬼狐, Demon Fox): In it's sealed form, Kikitsune takes the form of a regular katana with a diamond-shaped tsuba and white hilt and sheath.

Kikitsune's spirit, like Maryu's Kyūbimaru's, takes the form of a humanoid nine-tailed fox. Unlike with his brother, however, Kikitsune is a very calm-minded, serious and mostly silent Zanpakuto, only saying what he believes needs to be said. He is also fond of pushing Sengetsu to his limits as a Shinigami, even putting him in life-threatening situations to make him prove his power and ability. Kikitsune's outfit is pure white, and he constantly keeps a blue flame on the palm of his left hand. Out of the flame he can form his sword form and wield it, as he did during Sengetsu's bankai training.
 * Shikai: The release command for Kikitsune is let your fangs pierce the flesh of your enemy. Upon releasing shikai, Kikitsune transforms into a much bigger form. It takes the form of a big broadsword with the tip of the blade being sharply bent to form a kind of a hook. The blade is at the inner side of the hook. Sengetsu calls the hook "Kikitsune's fang" and usually uses it to pierce the enemy or to launch his special attack. At the end of the hilt there is a blue tassel.
 * Shikai special ability: Tenshōga (天衝牙, Heaven Piercing Fang). By charging Kikitsune with his reiryoku and swinging the blade Sengetsu can release a spear-like attack made from reiryoku. The spear is meant for doing large piercing damage on a pin-pointed location, but it can be quite devastating on a wide area if used correctly.


 * Bankai: Tenga Kikitsune (天牙鬼狐, Heavenly Fang Demon Fox): Kikitsune's bankai is often compared with his brother's as it too is small in size. It bares a great resemblance to Ichigo Kurosaki's Tensa Zangetsu in both size and ability, but Tenga Kikitsune is more focused on long-ranged techniques than pure melee fighting. Tenga Kikitsune is completely pure white, from blade to hilt, and the guard is a basic rectangular guard with no decorations. On the end of the hilt, there is a long silk cloth, much like Rukia Kuchiki's Sode no Shirayuki. The cloth is in constant movement due to Sengetsu's reiatsu. Sengetsu's outfit also changes with his bankai. His outfit becomes similar to Kikitsune's: pure white, and Sengetsu also gains nine "tails" formed of reiryoku, which function as supplies of reiryoku. Like Tensa Zangetsu, Tenga Kikitsune grants Sengetsu amazing speed due to its small size and focused spiritual power. Tenga Kikitsune has three special states with different powers:
 * Tenken: Senbon Tenshōga (天剣・千本天衝牙, Heavenly Sword: One Thousand Heaven Piercing Fangs): According to Sengetsu, Tenken is the name of the state Tenga Kikitsune enters upon it's released to Bankai. The special attack in Tenken state is a multiplied version of his normal Tenshōga attack: From the tails of reiryoku, several spears are formed and then blasted at the enemy. Sengetsu can also use a regular, single-shot version of Tenshōga while in Tenken state.
 * Ōken (王剣, Royal Sword): The second state of his bankai is named Ōken and can be entered by saying the name after his bankai is released. The Ōken state is similar to Maryu's "rip"-command enhancement: It focuses the reiryoku stored in his tails to the blade, making it glow white, greatly enhancing its offensive capabilities to the point of being able to create fissures to the ground with a simple swing of his sword. However, while Tenga Kikitsune is in Ōken state, it makes Sengetsu waste reiryoku in a much faster rate than normally. It also prevents the usage of Tenshōga. Sengetsu prefers to use the Kendō style of swordsmanship with this state of Tenga Kikitsune.
 * Shinken (真剣, True Sword): Tenga Kikitsune's most powerful ability, the Shinken state is something Sengetsu uses extremely rarely, as it is very costly. When beginning Shinken, Sengetsu announces the name of the state. In Shinken state Tenga Kikitsune begins to consume surrounding reishi with an alarmingly fast rate, comparable to Quincy Final Form. All the reishi Tenga Kikitsune consumes is focused in the blade, making it shine very brightly. Sengetsu is able to use the reishi he has gathered for a single attack with the command shine brightly for the last time. Upon saying this, all the reishi the blade has collected explodes with a violent blastwave, incinerating anything in range. However, afther the blast, the blade of Tenga Kikitsune shatters completely and Sengetsu is left unconcious from the loss of reiryoku. Once Shinken is activated, Sengetsu cannot deactivate it, meaning he has to finish the attack with it.

Trivia

 * Sengetsu is left-handed, which is also mirrored by Kikitsune's habit to keep an everburning blue flame on his left palm.