Zangetsu (斬月) is the manifested spirit of Ichigo Kurosaki's Zanpakuto that is seen during Ichigo's visits to his inner world, the anime-exclusive Zanpakuto Unknown Tales, and Beast Swords arcs.

Appearance:
The spirit of Zangetsu resembles a tall (about the height of Kenpachi Zaraki) 40 to 50-year-old man with unkempt facial hair and long ragged black hair. He dresses in tattered all black garbs with narrow sunglasses and a long flowing overcoat that flares out into ragged ends. Because of the spirit's appearance, Ichigo calls him "Old-man Zangetsu" (斬月のオッサン, Zangetsu no ossan) as a result. Zangetsu has also been called the "black-clothed man" (黒衣の男, kokui no otoko).In Bankai form, Tensa Zangetsu, he resembles a teenage version of himself and appears to be closer to Ichigo's age. He has long ragged black hair. He is dressed in a tattered, black hooded cloak that flares out into ragged ends. When Ichigo enters his inner world for the first time in Bankai, he finds that the skyscrapers in the inner world are submerged under breathable water.

Personality:
Zangetsu is a calm, level-headed Zanpakuto, in stark contrast to his hot-tempered master. He often acts as a mentor to Ichigo and always tries to push his master to become stronger.  While Ichigo's inner world is one of tall glassy skyscrapers, Zangetsu claims in episode previews to prefer more lush, natural surroundings, like bright, sunny skies, green forests and animals. Oddly the Inner Hollow professes to demand the same things as well. When Muramasa 'frees him of Ichigo's control, his reason for turning on Ichigo is very unique compared to the other rogue Zanpakutō spirits; while their reasons are all self serving, Zangetsu does not wish to leave Ichigo; he merely wishes to see Ichigo achieve his true potential and find out who is the stronger of the two.
In Bankai, Zangetsu seems to portray the same cool and calm collectiveness as he would when not in Bankai; however, he is far more aggressive. While fused with Hollow Ichigo, it seems that Zangetsu is the dominate personality, though he is prone to occasional outbursts like the Hollow once had. During Ichigo's Dangai training, Zangetsu explains that what Ichigo wants to protect is not what he wants to protect. He later reveals that what he wanted to protect was Ichigo himself.

Shikai: Zangetsu can summon a sword that takes the form of his constant Shikai release state: an oversized, elegant, black cleaver almost as tall as himself with no crossguard or proper hilt. It is instead held by its cloth-wrapped tang. Shikai Special Ability: Getsuga Tenshō (月牙天衝, Moon Fang Heaven-Piercer; "Piercer of Heaven" in the English anime): Zangetsu fires concentrated energy blasts from the tip of his blade in the shape of a crescent moon or in a shape of a wave.

Bankai: Tensa Zangetsu (天鎖斬月, Heavenly Chain Slaying Moon): It is considered completely out of the ordinary for any Zanpakutō. Unlike most Bankai forms, which usually create some sort of massive creature or effect, The Bankai actually shrinks his sword down to a daitō (Japanese long sword) with a black blade. The crossguard has four prongs bent out to form the shape of the manji (which is the kanji for "ban," meaning "full", as in "full release"). Instead of the cloth his Shikai form had, a short length of chain with a broken link at the end dangles from the base of the hilt.

Bankai Special Ability: Zangetsu's special attack remains Getsuga Tensho and can be used in the same manners, but can be used in an enhanced form. When used with Zangetsu's Bankai, these potent blasts can become black with a red outline and their path can be controlled, with explosively augmented power and speed. 
Physical Enhancement: This Bankai is a compression of his remarkable power, rather than a vast expansion as other Bankai are. This vastly compressed power enables to reach levels of physical and spiritual prowess most Shinigami would have their bodies give out at and cannot handle. It allows Zangetsu to move at very high speeds exceeding those of flash steps, allowing him to be undetectable to unfocused eyes. This Bankai also allows Zangetsu to use his natural strength to its limits. His new speed is so great, that it also allows him to create dozens of tangible clones to confuse his opponent. In addition, his already immense spiritual energy has grandly and explosively increased, and now has a deep black spiritual energy with a crimson outline.


Hollowfication:
Because Ichigo developed an inner Hollow (as a result from nearly becoming one while having his Shinigami powers restored), Zangetsu and the Hollow are both the source of Ichigo's power and share the same spirit body in Ichigo's inner world, the stronger presence retaining more control (and overall appearance) of their body; in which case, the spirit of Zangetsu takes the form of an all-white (clothes and being), somewhat deranged version of Ichigo's Shinigami form with a strong desire to take control of Ichigo's body and power for himself. The two entities can also fuse themselves into one body, revealing a body with Zangetsu's hair along with some features of the Hollow's new form.

Tensa Zangetsu and Hollow Ichigo fused into one.Hollow Fusion Form: After Tensa Zangetsu pulled Hollow Ichigo out of Ichigo during the latter's Dangai training, the two spirits proceeded to merge into one form, one that combined aspects from both spirit's appearances; Hollow Ichigo's white Bankai coat, black horn mask remnant, white-colored hair, and left Hollowfied eye, along Tensa Zangetsu's appearance, right blue eye, and black Bankai sword. In this form, Tensa Zangetsu explains that he and Hollow Ichigo were technically two parts of the same being with both of them together representing all of his spiritual powers. Ichigo claims that if the hollowfied Tensa Zangetsu really wanted to, he could've defeated him from the start of their battle, further saying they were that far apart in power.Enhanced Strength: While in this form, the hollowfied Tensa Zangetsu is strong enough to break Ichigo's Bankai sword completely in two.