Grim, is octopus, that resides inside the house of Port witches. It is used by Keeper Apprentices to proof their skills. They cut a piece of Grims tentacle and tries to return it back to Keeper Gottage.
In Syren, Wolfboy went to house of Port witches and told them he wanted to feed Grim and asked what would they give to him in order to feed it. Witches tried to give Lucy Gringe to him, but Wolfboy refused to feed Grim by her.
After witches went to upstairs, Wolboy told Lucy to scream and in meanwhile he feed Grim by cat and cutted piece of its tentacle, but forgotted to hide it and so witches smelled Grims blood and thinked that Wolofboy killed it.