Articuno is a legendary bird of frost, one could say, ice phoenix. Its history and purpose in folklore was almost the same as the Phoenix's, but instead of having the nature of fire, it had the nature of ice. It represented cleansing and rebirth as well, only from a slightly different angle. While the Phoenix represents a new beginning, Articuno represents a second chance, although the difference between these two concepts may not be understood by some
The bird's appearance is nothing really special: it has ice blue feathers, white fur on its chest and red eyes. There are also three " hair strands" of feathers on its head, blown back and of a darker blue color. It has a very long tail, which floats completely freely, something very unusual for any bird, and has the same darker color as its "hair"

Articuno's personality is hard to grasp. As expected, he is very cold to everyone, relying on his mind and logic much more than anything else. He is also very silent, speaks rarely, only when it is needed and always straight to the point. It could be shortly described as extremely serious, because there is no cruel or uncaring side to him, but he doesn't seem to be able to have fun

Articuno and Yuki's Avatar Form progresses over four stages, or five including the zero stage. As with everyone else, it doesn't do anything for her in Zero Stage, not affecting neither her powers nor appearance at all

1. Her First Stage is one of the rarest kind, one that is usually called "zero release", and brings with it some minimal changes to the girl's appearance, though they would catch one's eye easily: her eyes turn very light, silvery-blue in color and almost seem to have a glow to them
   
2. The Second Stage and the middle of her transformation, is the common "human avatar" form, where one gains the majority of their spirit's powers and some appearance traits. In addition to her eyes already being a pale blue, they really start glowing a little, and her hair turns the same ice blue color as Articuno's feathers. Her skin also turns slightly more pale and with a tiny hint of blue to it
   
3. The Third Stage is not a very common one, but this type is known as "hybrid form", where one's body becomes a hybrid between their human form and the guardian spirit. All the changes in appearance that appear in previous stages are still present in this, but in addition to them, Yuki gains Articuno's wings and tail, proportionated to her body's size: the tail is six feet long and her wingspan is about sixteen feet
   
4. The Fourth Stage is final for Yuki. A stage of this kind exists for every Hogosha, and is either called "final stage", "guardian form" or "spirit form". In this stage, the girl completely transforms, taking on Articuno's shape. She also grows in size, becoming about ten feet tall from claws to head, with a wingspan of about forty feet. Her tail can now change in length, from six to thirty feet, at her will, though it doesn't affect anything. She also has the ability to shrink in size, down to two feet tall, with her body parts shrinking accordingly
   

As it is always, Zero Stage has no effects whatsoever on Yuki

1. The First Stage allows the use of some additional powers, and it releases the limits of the human body, allowing her to use her full potential
   
2. The Second Stage allows the use of most powers, and boosts her Speed and Spirit Power by 20%
   
3. The Third Stage allows the use of all non-final form powers, boosts her Speed and Spirit Power by additional 30% (total of 50%) and allows her to fly freely
   
4. The Fourth Stage allows the use of all powers, but restricts the use of Guardian Arms and any human form only powers. It doubles all her stats, including the bonuses from previous stages. Since her maximum Spirit Energy and Endurance also double, her current Spirit Energy and Stamina get restored by the according amount (normal maximum). When she leaves this stage, her current Spirit Energy and Stamina fall by the same amount, but never below 0
   

If Yuki enters someone else's world, she will be on the same grounds as the owner of that world, meaning the possibly surreal and harmful conditions of it will be as natural for her as for the person themselves. The owner of this world will have their full Spirit Energy and Stamina when this happens, while she will only have the same amount as when she used this ability in reality. If she manages to kill them in their world, the person will die without any detectable effects on their body as their soul will be cleansed and brought back to the start of the circle of life. If she is killed in their world, she will be thrown back into reality and lose half of her maximum Spirit Energy. She is unable to leave their world at any time if she is in a fight with its owner
If she brings someone else to her Inner World, then she can control whether they are unaffected by its conditions like herself, affected partically or completely. When a person is brought into her world, they get fifty percent more Spirit Energy and Stamina than they had in reality, but no more than their maximum, while Yuki herself always has them at maximum. If the guest was conscious when brought there and Yuki kills them, they die without any detectable effects on their body, but if they were conscious, they are thrown back into reality and stripped of half of their maximum Spirit Energy. If she herself gets killed, the guest is thrown back into reality, but she is the one to lose half her Spirit Energy this time
Note that the events that transpire in reality and either of their Inner Worlds are not directly related, meaning the amount of Spirit Energy and Stamina each of them had in one world will only be affected by the other in the ways shown above. In other words, it does not really restore one's strength

In Zero Stage, she only gains minor ice and cold creation ability. She can form a piece of ice in her hand with a minimal temperature of minus four degrees Fahrenheit and can also channel cold of the same temperature through the air or any object over a distance of up to five feet, although it naturally weakens if the medium is warmer. When she channels cold, it is visible by distortions and tiny frozen water particles in the air, or frost on objects
In First Stage, her abilities to create cold and ice become stronger, allowing a distance of eight feet, a lump of ice twice the size of her hand and minus twenty two degrees Fahrenheit. However, anything beyond that drains from her energy. Additionally, she gets a "heat sense": somewhat similar to heat vision, except it has nothing to do with sight and allows her to feel the shape and temperature of all objects within one hundred feet from her
In Second Stage, Yuki's "heat sense" widens to cover an area of two hundred fifty feet from her, but that's all that she gains. She also doesn't get any increased common abilities in Third Stage, and her abilities change drastically in the Fourth (and final) Stage.