According to King Whitson Mariner, Lord Grimble hoped to become king of the rabbits, and was frustrated in this aim when King Gerard named Whitson as his heir. As such, though he joined the fleet Whitson led to Natalia, he acted in opposition to Whitson wherever he could. Whitson did his level best to reconcile with Grimble, even giving his son the rank of captain. Despite this, Grimble remained obstinate, though his efforts were opposed by such staunch allies of Whitson's as Mother Saramack.
At one point, Fleck Blackstar observes a whispered conversation between Lord Grimble and his son.
Aboard the Vanguard, King Whitson Mariner welcomes his lords and captains into his cabin for a toast. Whitson engages in a heated exchange with Lord Grimble and his son, Captain Grimble, prompting Queen Lillie to quell the rising storm by calling for a toast. Lord Grimble toasts to "a new kingdom", which so riles Whitson that Lillie is forced to take him out of the room before his temper can further elevate. Eventually, all the rabbits take their leave of their rulers, with Lord and Captain Grimble returning to Burnley to continue their journey.
Under Captain Grimble's command, Burnley failed to warn Vanguard of a rock that wrecked the vessel, and disappeared failing to answer hails. It is later revealed that the Grimbles have betrayed their king by leading him into a wreck on a mysterious island inhabited by dragons, with whom they have allied. Their plans are thwarted by the brave actions of Whitson and his subjects, and the dragons blame the Grimbles for this failure. Lord Grimble and a delegation of rabbits who were to meet with the dragons are then slaughtered-all save one-as punishment.
It is said that some time after the Battle of Ayman Lake, Lord Grimble led a number of followers in breaking away from Whitson's group. Some sources believe that Grimble stole the Stone Sword of Flint when he did so, and that it was later lost when the community he founded failed.
Personality and traits
Like his son, Captain Grimble, Lord Grimble was proud, self-centered, and cold-hearted. He cared nothing for the line of King Whitson Mariner, so much so that he was willing to kidnap Queen Lillie so as to promise his son an heir with a claim to the throne.