After becoming a council-recognized mage in his own right (at only 13!), Briar and his crotchety teacher Rosethorn start traveling their world, seeking new magics and spreading knowledge of their own. Briar has taken to cultivating the magical equivalent of bonsai trees, and while selling them in the bazaar he discovers a young street kid has magic. Convincing her to listen to him takes a great deal of effort--Evvy's as skittish as her bevy of cats--but eventually she agrees to be taught. The only problem is, the only stonemage in the area refuses to teach her. Briar is only a teenager himself, and his plant magic is utterly unlike Evvy's own stone magic, but seeing no either options he begins to teach her. But then a bored and power-hungry noblewoman starts a gang war, and Evvy has to work with her only half-realized magic to survive.
Briar is a goddamn badass
in this book. He's my least favorite of the Circle mages (only because I love Daja, Tris, and Sandry so hard--he's a perfectly cromulent character), and it's easy to think that the magic of living plants is too slow and subtle to be useful in battle. Not so. Briar tangles his foes in vines, tortures them with pollen (almost killing one of them), even sets a tree to eat a guard from the inside out.
He is a scary dude when he's riled. The action scenes in Pierce's novels are by far my favorite part, because as wonderful as her world-building and character development are, her action is just that much better. She is probably one of the best action writers working today.
Edited to add: also, Briar and Rosethorn's seed bombs are totally a real thing!