Sister Armel was a female squirrel Infirmary Keeper of Redwall Abbey. She was the wife of Rakkety Tam MacBurl and mother of Melanda MacBurl. She was described as very pretty, with beautiful, gentle, dark brown eyes.
Armel was a popular resident of Redwall, and respected by many, including the elders. She was liked by the Dibbuns and kept a jar of candied chestnuts in the Infirmary. Martin the Warrior appeared to Armel in a dream, informing her that she and Brookflow should take his sword to a Border warrior who sold and lost his sword. She and her otter friend left the Abbey immediately, but were unfortunately captured not long afterward by Gulo the Savage and his horde.
While captive, the pair was spotted by Yoofus Lightpaw, through whom Rakkety Tam MacBurl, along with his companion Wild Doogy Plumm and the Long Patrol, discovered the two Redwallers. The squirrel warriors set off to save Armel and Brookflow, during which Tam fell in love with Armel. Intrigued by her, she told him of the word from Martin. After saving the sword, the troupe returned to Redwall where a battle outside the gates ensued. Armel helped with fallen officers afterward, chatting warmly with Tam, to whom she herself had taken a liking to. Exchanging winks and teasing, the pair thought no one else noticed.
Armel was an excellent healer, working with both Brigadier Buckworthy Crumshaw and Tergen. She explained the significance and history of Martin's sword to Tam, and spent the remainder of his time at Redwall with him. On his departure, she gave him a satchel of herbs in case he was injured.
Afterwards, Armel consulted with the Brigadier about Tergen and also helped out with the Dibbuns during games and vermin alerts. Not a fighter herself, Armel never understood violence. However, she did her part to defend the Abbey. When a small vermin band entered the grounds while the Long Patrol was fighting, Armel distracted a white fox while Brookflow dealt him a surprise.
At Tam's return, Armel ran out to greet him where he confessed his love for her. They happily sat together during the feast and chatted. Tam was worried though about Doogy, and would sit on the walltops day and night as she would come to give him meals. However, their happy times soon ended when Tam challenged Gulo to a fight to the death on the flatlands. Armel watched from the dormitory windows the close battle between the huge wolverine and the relatively tiny squirrel warrior. A close battle, Tam survived and defeated Gulo.
Armel aided in bringing back Tam back to health from his injuries. While still healing, Doogy talked Armel into letting him have Tam's claymore. Ignorant of the importance of a sword to a warrior, she agreed. When Tam awoke, Doogy informed Tam of the news. Tam's anger turned toward Doogy. In the midst of the yelling, she ordered everyone out of the room so that she and Tam could have a spot of tea.
Tam and Armel wed and had a daughter. Seasons later, Armel finally persuaded Tam into returning the stolen Royal Banner to Squirrelking Araltum. Together with Tergen, Doogy, and Melanda, the four squirrels embarked on a journey to King Araltum's grove. There, Armel provided a moving speech simply ordering that the tyranny of Squirrelking Araltum and his queen end and that they all live in peace. She and her husband were offered the rule of the grove but they refused.