Mareth was a soldier in the Regalian Army and becomes a mentor and friend to Gregor. When Gregor arrived he is one of the two guards assigned to escort the Overlanders to the baths. Later, he saved Gregor and Boots on the beach when the rats attacked them. He pulled Gregor up onto his bond, Andromeda's, back. When the doctors wanted to put him on a stretcher, Mareth refused and insisted on helping carry his bond inside.
In Gregor the Overlander, Mareth accompanied the questers as a guard and was with them through the land of the fliers, crawlers and spinners. However, after Ripred emerged to guide the questers, Mareth left with Solovet, Vikus and their bonds.
In Gregor and the Prophecy of Bane, Mareth greeted Gregor warmly when he showed up for training. Mareth was training Gregor's age group and gave Gregor a short lesson in sword basics. He was also impressed when Gregor could keep up with him, both in speed and stamina, while running track. The group practiced tumbling and sword drills before each of them tried their hand at breaking the blood balls shot from small cannons. When Gregor raged for the first time, Mareth tries to call him back to the group, but he flees. Later, Mareth joined the quest to seek out the Bane. During the serpent attack at the Labyrinth his leg was severely injured and Andromeda and Howard decide to take him back to Regalia. When Gregor returns to Regalia, he finds that the three of them were facing trial for deserting the quest. They are eventually pardoned due to Gregor's testimony and Nerissa's revelation.
In Gregor and the Curse of the Warmbloods Gregor visits the hospital in Regalia to see his bond, Ares and his friends Henry and Andromeda who had the plague. Gregor ran into Mareth, and it was revealed that due to the severity of his injuries, his leg was amputated. Later, Mareth gave Gregor a backpack of supplies including duct tape, spring water and bubblegum. On the quest for the plague cure, Gregor met Hamnet, who was revealed to be the run-away son of Vikus and Solovet. Hamnet revealed that he knew Mareth, but doesn't explain everything at first. It is later revealed that during the battle at the Garden of Hesperides, Mareth pulled Hamnet from the water, saving his life.
In Gregor and the Marks of Secret, Mareth led the search party to find Gregor and Luxa within the palace. When Vikus and Howard found them "sleeping" in the old nursery (after their adventure in the jungle), Vikus said he would inform Mareth to call off the search party. Later, Mareth was the one to bring in the basket of nibbler pups that were found in the river.
In Gregor and the Code of Claw, Mareth greeted Gregor when he awoke in the Regalian hospital and asked if Gregor was okay. Gregor, who had just recently gotten news of his prophesied death, said he was fine. Mareth gave Gregor the picture of Gregor and Luxa dancing, but did not comment on it. Mareth revealed to Gregor that Solovet was still in charge and had ordered that Gregor stay in Regalia. Gregor then broke out and goes to the battle anyhow. When Gregor returned with Luxa, who was gravely ill, Mareth was the one who took care of Gregor. He and Gregor then go to help with the nibbler pups who were being evacuated, where they become separated.
Mareth was in the war council room the next time Gregor saw him and was surprised and horrified to learn that Solovet locked Gregor up in the dungeon for his disobedience. To keep Solovet from locking him up again, he told her to check Gregor's pocket, where he keeps the picture of Luxa and himself. Gregor feels betrayed that Mareth revealed his feelings about the queen; However, Mareth argued that Solovet once locked Hamnet up for a month when she crossed her at a war meeting, and would not hesitate to lock Gregor up until he would obey her.
The rats arrived at the walls of Regalia and Gregor and Ares joined the fight. After the first wave of attacks, Solovet called Mareth and Perdita from the battle to retrain Gregor and Ares.
Later, when Solovet died, Ripred lamented that Mareth was in Regalia and unable to command. This implied that he would have been the natural successor. Instead, Perdita became the head of the army.
When Gregor came to after the final battle with the Bane, Mareth explains that Ripred died in the fight. Mareth was present at the surrender and was part of Luxa's support system. He was chosen as the human ambassador to find a peaceful solution along with Nike the flier, Temp the crawler, Heronian the nibbler, Lapblood the gnawer, Treflex the spinner and an unnamed digger.