Briar Moss was, in early life, a street rat called 'Roach'. In The Circle of Magic quartet, he is about 10; in The Circle Opens quartet, he's 14; and in The Will of the Empress, Briar is 18. His mother was killed when he was approximately four, and he was recruited by a gang, the Lightnings, led by the 'Thief-Lord', in Deadman's District in Hajra. He was caught three times, receiving a tattoo of an X on the webbing between each thumb and forefinger for the first two; on the third, he was sentenced to life at the docks. A great mage, Niklaren Goldeye, had a vision that he would find special mages—mages whose ambient powers were not easily discernible by the usual means—and as such was there, as he needed to be, during Briar's trial. He was able to detect the boy's psychic affinity with plants, and took him to Winding Circle Temple to be educated in plant magic.
Although initially a thief, Briar developed a strong sense of right and wrong, which would serve as a weapon against criminals and seductresses. He is cantankerous but possesses a profound loyalty to his foster-sisters Daja Kisubo (whom he calls Daj'), Sandrilene (Sandry) fa Toren, and Trisana (usually called Tris for short, but Briar often calls her Coppercurls) Chandler and his teacher Rosethorn, as well as a cutting sense of humor. His affinity with all things botanical has made him not only a master gardener but has also led to such oddities as stealing and later coming to own an ancient, magic-storing, miniature pine tree called a shakkan, tattoos of flowers that live and grow under his skin (he attempted to tattoo over his prisoner X tattoos with vegetable dyes and one of Sandry's needles), and the ability to control growth of plants. During the Circle Opens, he is shown to be somewhat flirty, but in the Will of the Empress he becomes much more promiscuous. In the same book, he was suffering from posttraumatic stress disorder.
Briar has plant magic and his teacher is Dedicate Rosethorn, who is also a plant mage. Briar can make plants grow very quickly and move them to any form, like creating a snare from a vine. Rosethorn taught him how to make medicines and restore them to their full potential when they had expired. He also enjoys and makes money by caring for shakkans (similar to bonsai), trees kept small which usually store magic in the way the plant looks. He and Rosethorn developed special weapons to use in combat. They are seeds, namely of thorny plants, wrapped in cotton wool. Thrown, Rosethorn and Briar use their green magic to simultaneously cause the wool to unwrap and the seeds to grow. There are variations of the balls, some containing vines to wrap around people and others to do actual harm. When trying to tattoo over his "X" marks, Briar's green magic and Sandry's magical needles resulted in a subcutaneous garden; plants grow and shift beneath the skin on his hands, often reflecting his mood. This could be considered Briar's "unique ability" (as with Sandry's power to spin pure magic, Daja's living metal, and Tris's scrying ability and sheer versatility), though these plants have yet to show any practical magical properties.
In Briar's Book, a deadly epidemic threatens everyone in the city. His teacher, Rosethorn, catches "blue pox", as it is called, has a seizure and dies. Briar, with the help of Sandry, Daja, and Tris, goes to the afterlife to bring her back to life, almost dying himself.
In Street Magic, Briar finds Evumeimei Dingzai (Evvy), a former slave, who has stone magic. Evvy refuses to go with the nearest stone mage (Jebilu Stoneslicer) and thus, Briar must become her teacher. Briar teaches her meditation and at the same time about rocks and what they are used for. Evvy calls Briar's plants 'stone killers' because their roots dig into rocks and destroy them over time. In Melting Stones she revises this attitude slightly, believing that rocks occasionally welcome the change wrought by plant roots. In The Will of the Empress one of his foster-sisters asked him if and who was the girl he cared for more than other girls to bring to their breakfast, and Briar answers, Evvy.