Puddleglum is a gloomy, serious Marsh-wiggle from the Eastern Marshes who serves as Eustace and Jill's guide and companion on their journey to find the missing Prince Rilian. He initially agrees to join the quest because he is considered "flighty" by all the other Marsh-wiggles, and thinks an expedition with little chance for success will be just the thing to sober him down.
He always looks on the worst side of things, and Eustace and Jill often refer to him as a "wet blanket". But despite his depressing attitude, he has a heroic heart and means well. He's an excellent guide and used to living outdoors, and is an immeasurable help to the children as they travel across the dreary plain of Ettinsmoor. He's very cautious and suspicious, and when the travelers meet up with a strange, beautiful woman and her silent companion, he senses that something is not right and refuses to give her information about their quest, even though the two children would have gladly volunteered all the details without a second thought.
When they come across the city of the giants, Puddleglum wants to keep going, but reluctantly agrees to stop there for a while when Jill and Eustace beg him to be sensible. He gets a bit inebriated on a drink that is offered to him by one of the Giants, and wakes up the next morning with a terrific headache. His distrust of the giants is well-founded when they realize that the Giants are not the kind, gentle folk they thought them to be. The Giants feast on Talking Animals, and indeed intend to eat the three travelers the following night at their annual Autumn Feast. When they realize the danger that they're in, they manage to escape the castle but are pursued by hounds. In their haste to get away, they stumble down a hole that leads them to the Underworld, where they are taken captive by the Earthlings and are brought to see the Prince of the Underworld.
When the strange Prince is revealed to be Prince Rilian, Puddlegum's loyalty and faith in Aslan is put to the test when the Queen of the Underworld returns and attempts to put a spell of forgetfulness on the four Narnians. Puddleglum is the only one who can resist the enchantment with his unwavering faith and successfully breaks the spell by stamping out an enchanted fire with his bare foot. When the Queen turns herself into a serpent, he along with Eustace and Rilian work together to slay her.
The spell of the Underworld forever broken, they all are able to escape the hidden country and return to Narnia. There, Puddleglum is treated for the burns on his foot and is returned home, but not before Jill bids him farewell with a fond hug and kiss. He's surprised and delighted by this demonstration, and shows a degree of vanity in himself that had previously not been made aware.
Puddleglum lived a good life, and was present at the Great Reunion in Aslan's Country.