|I am a cat.|
|— Grim to Meghan Chase in The Iron Queen, pg. 292|
While being chased, Puck, as a horse, bucks Meghan off his back and into a tree where Grimalkin was, having watched what had happened. Grimalkin decides to help her find Puck only after she promises to repay the favor. Meghan has a hard time following him and, as such, is captured by goblins and put in a cage. While Meghan distracted them, Grimalkin chewed at the cage to free her. When she is out, they run when the goblins attack.
At the Seelie Court, Grim tells Oberon that Meghan was in his debt. When he leaves, Tansy explains to Meghan that Oberon had taken Meghan's life debt to the cat and made it his own so Grim couldn't ask anything of her. Meghan doesn't see him again until Elysium, where he explains some of the creatures to her. After they are attacked by a Chimera, Grim is willing to take her to the Unseelie Court to look for her brother, but he tells her this time his favor would be large and she accepts.
Grimalkin has no apparent romantic relationships, though he does have several friends, allies and acquaintances. He first befriends Meghan when he helps her find Puck when the two got separated. After, he becomes allies with Puck and Ash. In the Iron Knight, Grimalkin and The Wolf become acquaintances. They are forever enemies, but become allies for the sake of Ash.
He is very wise and mysterious, often disappearing without warning in the face of danger. He always appears, however, at the last moment in order to save everyone. Grimalkin can be very patronizing and impatient with dim witted people, but he is very informative and helpful all the same. There are many people who owe him, and he often smug when collecting his debts. In the iron knight the group faces reflections of themselves. While the others look at their reflections and they look different and harsh, Grim's is the same as his physical form. He explained to them that the reflections showed who they truly were or could be. Because he is already his true self
Grim is a huge cat with long, whispy dark gray fur, a plumed tail, and golden eyes. Meghan describes him as a slightly plump house cat. It is suspected he is a Persian cat, very similar to the one shown in the photo.