What is Pooka?
Pooka is Pooka! If you're not tired of hearing this yet, I haven't done my job!
In all seriousness, though, the Pooka is a Celctic trickster that can change it's shape. It is usually seen as a malovent creature that takes the form of a large black dog, horse, or rabbit. The Pooka likes to trick people and remain untamed. The Pooka also loves to chat! They will talk and talk with you but then leave whenever they feel inclined to do so. In most cases, one would not even know that they had been conversing with a Pooka until they disappeared. The Pooka is a feared creature and known trickster, but that does not mean it is without some benevolence.
Other than sharing a name and the ability to shape shift, our Pooka and it's possible namesake share little else in common. In Wild Arms 2nd Ignition Pooka is not black, large, or malevolent. Pooka is small, pink (cuddly), and I would say neutral in terms of good or bad. Much of Wild Arms is based in perspective as what may seem bad for one group is good for another. Pooka's role is ultimately to lead Tim into his role as the Pillar. Tim accepts the majority of this role. Tim chooses to save the world on his own terms, Pooka supports this. He convinces the Guardians that he will help Tim do this. The Guardians allow him to do this. The guardians are thought of as a usually benevolent force, so it could also be argued that in general Pooka is benevolent too. With that being said, Pooka does not openly play pranks or trick people. He does withhold information as part of his initial role but I do not believe he has malicious or detrimental intent when he does so.
Perhaps the reason that Pooka is so small and cuddly is because the Pillar is a twelve year old boy. If Pooka can shape shift, he could be anything that he needed to be. It would stand to reason that he (or the Guardians) would choose a form that suited the needs of the Pillar. I doubt that Tim would have approached a large, wild, dark-furred animal. That would have been detrimental to the Guardian's cause. The game does not specify if Pooka can change shape without mediums or that he even has the desire to do so. Pooka merely states that through his abilities he can join with equipped mediums and give physical form to the sleeping Guardians.