Arlo Armadillo is a Mobian western armadillo and was a member of the Substitute Freedom Fighters. He was personally trained by Sally Acorn and voiced his confidence in the princess' abilities during his training.
During the mission to attack one of Dr. Ivo Robotnik's energy plants, Arlo's hand glider was struck by an anti-aircraft turret and he crashed. While Penelope Platypus remained by his side, the rest of the team, with help from Geoffrey St. John, proceeded with the mission and destroyed the base. While the team was away, Arlo and Penelope were discovered by Royal Underground agents Fleming and Smiley, who moved them to safety and provided Arlo with medical treatment. Arlo and Penelope rejoined the group of trainees during their attack against another of Robotnik's bases, which saw the destruction of the Sally Auto Automaton. (PSM: #1, #2, #3)
Some time later, Arlo helped to defeat Sonic when he had lost his memories and was convinced by Dr. Ivo Robotnik to attack the Knothole Freedom Fighters. Attacking Sonic by himself, the blue hedgehog said "I'll say this for you kid - you have more courage than sense!" before tossing Arlo into Penelope. The distracted Sonic was then knocked out by Dylan and awoke later with his memories returned. (StH: #28)
Later, Arlo helped rescue the Knothole Freedom Fighters from Robotropolis after the group was captured by Combots. He and his teammates officially became a group under Larry Lynx when they saved the Knothole Freedom Fighters from Robotnik on yet another occasion. (StH: #38, SLV)
After the defeat of Dr. Ivo Robotnik, Arlo joined the other Freedom Fighter groups in retaking Mobotropolis. While exploring an underground region of the city, part of the roof caved in and fell on Arlo, leaving him trapped. Penelope called out for help, which got the attention of both Sonic and Tails. Although Sonic was unable to free him, the Robian polar bear Rudyard showed up and managed to remove the debris, freeing Arlo. However, the incident injured Arlo to the point where he was forced to retire from active duty. (SSS: #2, CSE)
Over a year later, Arlo was present at the wedding of Antoine and Bunnie D'Coolette. Like the vast majority of the Kingdom of Knothole's population, Arlo was subsequently captured and sent to an Egg Grape Chamber during the Destruction of Knothole, but was liberated shortly afterwards by Sonic and a few others. With his team disbanded, Arlo was left a civilian resident of New Mobotropolis, while most of his former teammates joined the Council of Acorn or the Secret Freedom Fighters. (StH: #174, #175, #176, CSE)
Despite possessing ordinary Mobian abilities, Arlo was a dedicated Freedom Fighter and brave combatant in the war against Robotnik. He shared a strong bond with his teammates, particularly Penelope, whom he has some feelings for. (CSE)
- Strangely, although Arlo shares his species with Mighty the Armadillo and Matilda the Armadillo, he has a completely different design. The Complete Sonic Comic Encyclopedia later explained that this is because Arlo is a Western Armadillo while the others are Eastern Armadillos. This is also a nod to the fact that Arlo is native to Northamer, which is west of Mighty and Matilda's homeland of Mercia.