Deba no Kyojin is sealed as an umbrella. A sword is hidden in the umbrella.
When the Shinigami utters the release command "Enlarge", the umbrella glows and morphs into a truly gigantic sword.
Ushiko is able to change the weight of Deba no Kyojin on a whim, able to make it drop from several tons to almost weighing nothing and back again. Lifting such a large sword gives Ushiko the illusion of super strength, but with the weight-changing powers, Ushiko only needs average strength to wield the sword. By making it weigh almost nothing, Ushiko can lift and swing it with ease. But she can rapidly and drastically increase the weight of the Zanpakutou at the second before the sword hits to land a devastating blow. The large sword can also act as a shield, as it can become so heavy and so dense that it can easily block attacks. Another benefit is that the sword is extremely heavy by default, and only Ushiko is able to change the weight. If an opponent tries to wield Deba no Kyojin, they will be unable to lift it due to how heavy it is.
The Bankai is called Tadaigane Deba no Kyojin (lit Heavy Metal Giant's Knife). With the command "Bankai", a large explosion of light engulfs Ushiko. When the light settles, Ushiko is standing in an intricate suit of armor.
The Bankai seems to be an emphasis of the clothing alterations that commonly occur when a Shinigami releases Bankai. But this suit of armor is not just for show like the other clothing alterations of other Shinigami's Bankai. Just like the Shikai sword, Ushiko can alter the weight of the armor on a whim. She can make it weight several tons, or make it light as a feather, and swap the weight from one extreme to the next very quickly. She can make the armor weigh next to nothing so she can move around, but she can also make it very heavy so she can deliver devastating bunches and body blows. Increasing the weight also has defensive purposes, as she can make the armor so heavy and so dense that it can withstand any attack, be it physical or energy-based.
The armor has one upgrade of the Shikai. Ushiko's body's own weight doesn't effect the weight of the armor. That means that when she alters the weight to make it weight next to nothing, it really will weight next to nothing. Her own body's weight is fully contained within.
Ushiko has mastered a few techniques with her Bankai.
Tremor Stomp: Ushiko drops the weight of the armor and raises her leg. She then sharply increases the weight and lets her leg drop to the ground and stomp it, creating a tremor that knocks the opponent off balance.
Cape Spin: Dropping the weight of the armor, Ushiko spins in place and moves about the battlefield, the cape of the armor flowing up and swinging around her as she spins. When she nears the opponent, she sharply increases the weight so that the heavy cape smacks the opponent in the face. At higher weights, it can crush the opponent's skull, but usually Ushiko just puts enough weight in it to break the opponent's nose and/or dislocate their jaw.
Rending Earth: Dropping the weight of the armor to barely nothing, Ushiko jumps high above the opponent. Then when she is directly above, she sharply increases the weight to several tons, dropping to the ground and landing directly on top of the opponent, creating a large crater and likely crushing the one under her.
Deba no Kyojin's spirit is a gigantic cycloptic troll-like giant the size of a small mountain. It speaks slowly, but it is not stupid. It wears a proportionate apron with just as large tools in it, suggesting it is a blacksmith.