Frodo is one of the most regularly-appearing characters in Moonshadow, although he does not take part in many major plot points. He usually appears as Moonshadow's silent companion and emotional support. He could be seen as something as a totem item for Moon, a symbol of security, family, and mystery, along with Moon's flute.
Major Story Arcs
Frodo begins life as a feces-covered cat belonging to a hippie commune. One night, Sunflower is holding him when they are both abducted by the alien G'L-Doses and transported to the Zoo, a benign location in space where creatures from across the universe have been brought to fulfill the whim of the G'L-Doses.
Sunflower and Frodo live together alone until the birth of Moonshadow (his father is a G'L-Doses). Frodo becomes Moon's constant companion from then on.
When the G'L-Doses eventually let Moon leave the Zoo, they send Frodo, Sunflower, and another friend, Ira, along with him. They travel through space and have many adventures. Frodo stays close, walking along behind Moon or even riding inside his space helmet. He follows Moonshadow into the insane asylum, his attempt to rob the Godsegg, and jail. When Moon went to war, Frodo is absent, but suddenly appears (possibly through the invervention of the G'L-Doses) when Moon finds Ira and is captured.
In the prisoner-of-war camp, Frodo breaks away to hunt, and when Moon goes off to look for him, he runs into Darkmeister Ebann, the head of the camp. Ebann takes a shine to Moon and helps him for a while. However he eventually sentences Moon and his friends, including Frodo, to death. They are saved by the G'L-Doses and becomes heroes of the realm. They live with the king and queen and nanny their children before being forced out.
They find a happy life on Pillbox before searching for and finding the missing Ira. When they are captured by Jobidiah Unkshuss's men at IHOT, and brought to a dungeon on GimmeGimme, Frodo seems to be absent. However, he once again appears after Moon and Ira are freed.
When Moon goes to the World of Mists, Frodo is his sole companion during his Awakening, everyone else having left or died. They leave together, dancing to a William Blake tune piped on Moon's flute.
As seen throughout the flashforward scenes in Moonshadow, Frodo remains with Moon in his old age. This continues in Farewell Moonshadow.
Moon, now a young adult, and Frodo, now much better cared-for than he was long ago on the commune, go back to Pillbox. He and Moon sit together on their porch, piping on Moon's flute, or working in Moon's tinker shop. Eventually the find Bettina, Moon's long-lost love, and he marries her. In one of the few moments of true characterization for Frodo, he is described as mewing at the moon with "insufferable longing" at night, just as Moonshadow himself feels.
A storm comes and kills Moon's new family. Only Frodo remains, with a broken leg. Moon considers killing him out of despair but instead hugs him, once again his only companion. Eventually Moon finds peace as a provider for the poor and bereft in his area. A mythology springs up about his life, and he becomes famous across the universe.
Many years later, Moon is a very old man in a wheelchair. Frodo remains on his lap as they celebrate Moon's birthday. He recalls his life as a young adult, then dies. He is then resurrected as a G'L-Doses himself (like his father). Frodo, still his truest companion, floats on top of him as they soar off together into the heavens, where they merge with the moon.
All members of the Zoo are given immortal lifespans when they are in the Zoo. This seems to wear off for the others when they leave the Zoo (for instance Moonshadow eventually dies of old age), but Frodo seems to retain his immortality, living until Moon is an old man, far beyond a cat's normal lifespan.