Aava appears to be a saber-tooth tiger. It was one of the seven beasts who served the Ivory King. Each of these beasts was given a specific task. We know of only 3 beasts that remain; Aava, Lud and Zallen. Aava’s task was the guardianship of the King’s beloved child of Dark; Alsanna. Lud and Zallen were tasked with the mercy killing of exiles. From this we know that the beasts were given a broad range of tasks, from protecting life to taking it.
Since the Ivory King was served by 7 of these massive beasts, it indicates he must have been very powerful. Aava is invisible to the naked eye. The only way to actually see Aava is to look through the Eye of the Priestess, an eye of the first priestess who watched over Eleum Loyce. These priestesses worked to contain the Old Chaos but by the time the Player reaches Eleum Loyce, the line of priestesses has been extinguished.
Aava’s soul can be crafted into the Ivory Straight Sword. What connection Aava may have to Sir Fabian is unknown, as Sir Fabian was said to have delved into Chaos in an attempt to exterminate the horrors that dwelt there.
Aava appears to have control over ice, as it can produce ice pillars. This ability is similar to that of Alsanna who can create ice to protect her, and calm the blizzard that swirls in Eleum Loyce.
Why there were 7 beasts is unclear. It may be that it is simply a thematic connection to the 7 Daughters of Chaos, as the eventual fate of the Burnt Ivory King seems remarkably similar to the Bed of Chaos (the Witch of Izalith). We are even asked to end the life of the Ivory King by Alsanna, which reflects the request of Quelaana for us to kill her mother, the Bed of Chaos.