Yrael has taken many shapes, a few of which are known. His most prevalent form in the trilogy is that of a small white cat, which is apt since he is rather cynical or 'catty'. He is also seen as a short waisted albino man with green eyes and incredible strength. Much like an ant carrying more than it's own weight, he showed his strength in Abhorsen when he picked up Nicholas Sayre. Regardless of his form, he mainly loves to eat fish, be an annoyance and help at the very last moment. When freed the second time in Sabriel he is rebound by Ranna because Saraneth was destroyed. Essentially, he was bound with the Sleepbringer instead of The Binder - despite Ranna being weaker. However, Ranna is said to "hold tightly when she must". Thus he is always very sleepy, demanding to be carried around in arms, or usually a pack (e.g. throughout Lirael and Abhorsen, he rides in Prince Sameth's pack). He is also able to use the miniature Ranna on his collar to induce weariness in others.
Of course, despite his murderous tendancies, there is some good in this cynical cantankerous creature: in Abhorsen Prince Sameth removes his collar at the last moment of the Binding of Orannis, thus freeing Mogget and returning him to his pure Free Magic form. This ultimately saves the world, as instead of Seven binding the feared destroyer Orannis, there are Eight. Mogget chose to be neutral in the beginning, but he rescinds his actions, choosing more wisely for: "Life. Fish and fowl, warm sun and shady trees, the field mice in the wheat, under the cool light of the moon." He proves to like life more than the power of destruction thus saving the Clayr, Wallmaker, Abhorsen, and Royal bloodlines present, as well as the rest of the world and universe which Orannis would destroy.
As due reward, Sabriel frees Mogget in the end with Belgaer, giving him the choice of will, to do as he pleases, hopefully for good; releasing him of his servitude to the Abhorsen forever more.