Weapon and abilities.

All shinigami in the world of Bleach carry a weapon known as a zanpakutou. This weapon is traditionally in the form of a sword, though the types of blades can vary. These weapons have souls of their own, and are therefore entirely unique, unlike mass-produced regular blades. They each have their own powers, influenced by their owner, and are capable of changing form depending on their wielder. Hitsugaya's zanpakutou is called Hyourinmaru, and can take multiple forms.
Hyourinmaru, "Ice Ring," is the strongest ice-elemental zanpakutou in all of Soul Society.
In its sealed form, Hyourinmaru takes the shape of a katana with a total length of 1.4 meters (around 54 inches/140 centimeters) which effectively makes it taller than Hitsugaya himself; he is forced to wear it strapped to his back, and even then it look very cloe to dra. Unlike traditional katana, Hyourinmaru's handguard is shaped like a four-pointed bronze star. The hilt is light blue and the sheath, which disappears when the blade is drawn, is dark blue in color.

A zanpakutou's sealed form is the standard, the first form a zanpakutou will take. However, this is not its only form. A shinigami will have greater access to their weapon's power after learning its name. Because zanpakutou have individual souls, they already have name of which must be discovered by their shinigami. In order to do so, a shinigami must be able to converse and connect with their zanpakutou, which involves entering an internal world where they can communicate with a physical embodiment of their zanpakutou. By learning their name and kaigo, or a unique "release incantation," the zanpakutou's physical form will change and its wielder will become even more formidable in combat. This form is called shikai, or "initial release."

❝Sit Upon the Frozen Heavens.❞
When Hitsugaya speaks the above release command, Hyourinmaru changes to its shikai form. In this form, the blade becomes slightly longer (perhaps to compensate for Hitsugaya's shorter reach? Just don't say that to him.) and forms a small, crescent-shaped secondary blade. This blade is attached to the hilt of his zanpakutou by a long metal chain. The chain can be used to attack or trap an opponent, and its length can change according to Hitsugaya's will.

Once Hyourinmaru is in shikai form, Hitsugaya has access to several different abilities involving the manipulation of ice and water.

Ice manipulation: With a slash of his sword, Hitsugaya can create dragons from ice, singularly or in great numbers. These dragons are a moving extension of his blade that he can fire at opponents, freezing anything they touch. They can also be manipulated to attack from multiple angles at once. He can also form a kind of tidal wave of ice that sweeps over and freezes opponents. In addition, the chain-blade at the hilt of his sword can freeze whatever it entraps.

Tensou juurin: "Subjugation of the Heavens." Hyourinmaru's most powerful ability in shikai form. It allows control over the water in the atmosphere, which also allows control over the very weather. His power over the water in the atmosphere is so great that the simple act of shikai release is enough to cause a blizzard in the Soul Society.

Rokui hyouketsujin: "Six-Clothed Ice Binding-Array." An ability that can be used to immobilize an enemy. Hitsugaya lays down five snowflake-shaped ice crystals on the ground in the shape of a pentagon. Once the opponent steps inside, a trail of ice emerges from each crystal and a pillar of ice forms, effectively trapping the opponent.

Ryoujin hyouheki: "Woven-Ice Wall." Hitsugaya can create a protective wall of ice using thin strands of ice in a sort tapestry-like construction.

Shinkuu tasou hyouheki: "Multilayer Vacuum Ice Wall." When unable to use his bankai, Hitsugaya developed this ability that makes use of his limited quantities of ice and the zanpakutou of his lieutenant, Matsumoto Rangiku. By combining the ash that constructs her zanpakutou and the ice from his, he can fabricate a multi-layer wall of ice with empty layers inside of it vacated by Matsumoto's recalled zanpakutou. This makes his ice wall more defensible against fire-based attacks.

Shinkuu no koori no yaiba: "Vacuum Ice Blade." Hitsugaya can utilize the technique developed above to gather the air-pocketed ice and create a large sword from it with which to impale his opponent.

After achieving shikai, a shinigami's zanpakutou has the potential for another form known as bankai, or "final release." This form can be achieved by learning the rest of a zanpakutou's full name, and is exceedingly difficult to master. The average time it takes one to achieve bankai is ten years, and it is a requirement (with one exception) for all Captains to have. Hitsugaya is the youngest shinigami to have ever achieved bankai, and therefore the youngest to ever become a squad Captain.
❝Daiguren Hyourinmaru.❞
Hyourinmaru's full name is Daiguren Hyourinmaru, or "Grand Crimson Lotus Ice Ring," which is a bit of a mouthful. When Hitsugaya speaks this name, the true form of his power is revealed. Ice forms a pair of wings, attached to his back and spreading over his shoulders. He gains a tail of ice, and more ice forms on his hands and feet, giving him claws. His dominant arm is covered in ice shaped like the head of a dragon, encasing his hand and the hilt of his zanpakutou. Each of these components are mobile, extensions of himself just as his flesh and blood limbs are. The handguard of his sword becomes an eight-point brone star instead of its usual four-point shape. Finally, three flowers of ice float in the air behind him, each with four petals, that diappear over time as his battle progresses. His enemies and allies assume they are a counter and indicate some kind of limit to his bankai.

With his bankai released, Hitsugaya's power increases exponentially. His power over the water in the atmosphere is so great that he would rather not use his Tensou Juurin in his bankai state, for he could potentially kill everyone around him with it. He would prefer to use that ability only with his shikai release to minimie damage. Compared to his shikai form, the amount of ice he can create is greatly increased, and his ability to freeze objects and living creatures is increased along with it. He can even swing his sparkly new tail around and freeze whatever it touches. Aside from these enhancements, Hitsugaya's bankai allows him access to stronger abilities.

Ice wing shield: Aside from actually flying with them, Hitsugaya can use his ice wings as a shield, wrapping them around himself to protect from an opponent's attack.

Bankai regeneration: As long as there is water in the atmosphere, Hitsugaya can regenerate his bankai as he sees fit.

Zanhyou ningyou: "Lingering Ice Doll." Using his masterful control over ice, Hitsugaya can reshape it into an exact copy of himself, even going so far as to make it appear to bleed. He usually saves this move for the precise moment where his opponent is least on guard, as his opponent likely won't be fooled by it a second time.

Ryuusenka: "Dragon Hail Flower." Hitsugaya can activate this ability the moment he strikes an enemy. Ice immediately forms from the point in which he hits his opponent, encasing them in ice that he proceeds to shatter, along with the one inside.

Guncho tsurara: "Icicle Flock." Hitsugaya creates daggers of ice, then sends them flying at his opponent with a swing of his sword.

Hyouryuu senbi: "Ice Dragon Swirling Tail." Despite its name, this technique has nothing to do with his bankai form's tail. Hitsugaya swings his sword and creates ice from the tip of his blade in a crescent shape.

Zekku: "Void Sever." An extension of the above technique, where Hitsugaya controls the flow of the ice to send it vertically into the air.

Sennen hyourou: "Thousand Years' Ice Prison." A move where Hitsugaya creates multiple ice pillars surrounding his opponent. At his command, these pillars rush toward the enemy, effectively crushing them. This ability appears to be somewhat intricate, as it takes some time to prepare and his opponent needs to be distracted while he does so.

Hyouten hyakkasou: "Frozen Heaven Hundred Flower Funeral." An extension of his Tensou Juurin, this technique controls the weather to create snow. When these snowflakes touch his opponent, flowers made of ice sprout on contact until the opponent is completely encased in an ice pillar. He states that when the last of the one-hundred snowflakes fall, the target will perish.

Generally, a shinigami's bankai is the final form of their power and weaponry. However, both shinigami and zanpakutou have their own souls, and the thing about souls is that they are ever changing and evolving. Kurosaki Ichigo's fluctuating power is the biggest example of this in the series. His powers are never constant, are always changing and improving. He isn't the only one, though. Throughout the course of the series, it is assumed by enemies and allies alike that the ice petals floating behind Hitsugaya in his bankai form are a timer of sorts, an indicator of how his bankai has not reached its full strength due to his weakness. Near the tale's end, we discover that this isn't the case. The petals were acting as a timer, certainly, but were a countdown to when his bankai would reach full maturity, rather then its limit.
❝Daiguren Hyourinmaru... has reached full maturity.❞
When the last of the ice petals disappears, Daiguren Hyourinmaru reaches its most "mature" form. I hesitate to say "completed" because as long as humans are alive, nothing they do is ever truly complete as they are always changing. Daiguren Hyourinmaru might have become more "mature" but that doesn't necessarily mean it is "complete." This improved Daiguren Hyourinmaru changes Hitsugaya's form into that of an adult version of himself. He speculates it is because his younger body is unable to handle the power of a matured bankai, so Hyourinmaru compensates for that by temporarily aging him. The giant ice wings, tail, and petals are nowhere to be seen, and the ice on his body now appears to be more like armor rather than extensions of his body. He has what looks like pauldrons of ice on his shoulders, bracers on his arms that reach his elbows, and guards on his lower legs that reach his knees.

This form appears to be very taxing on Hitsugaya, as he is perhaps not supposed to wield it until he is actually an adult. It is more powerful than the Daiguren Hyourinmaru we have previously come to know, with a couple of new and stronger abilities accompanying the change of form.

Flash-freezing: A basic, unnamed, and extremely powerful addition to his icy repertoire. When his power is matured, Hitsugaya has the ability to flash-freeze anything, even large and/or distant objects, with a simple wave of his hand. This ability extends to anything that touches him or his sword. Anything that is frozen will also have its functions or abilities completely negated.

Shikai hyouketsu: "Four World Freeze." The ability to freeze any matter in front of him completely within four seconds.

These are the last improvements we see in Hitsugaya's abilities before the end of the series. But his powers still have room to grow, without a doubt, and he is the kind of person to constantly push himself to evolve further. I am sure that even beyond the events of the main story, Hitsugaya will still work to improve his abilities in order to best protect the Soul Society and the ones important to him.