"Spare us ...The brats of Mudbloods do not stir our sympathies."
-- Yaxley (DH13)
Yaxley is a Death Eater with a prominent role during Voldemort’s second rise to power. According to Snape, Yaxley denied Voldemort to escape Azkaban when Voldemort disappeared after attacking the Potters (HBP2).
Yaxley is the “brutal-faced Death Eater” present at the Battle of the Tower. He is one of the four Death Eaters who break through the Order’s ranks and make it to the top of the Astronomy Tower (HBP27). He is the most focused of the Death Eaters there – unlike the Carrows and Fenrir Greyback, he does not engage in conversation with Dumbledore. Rather, he encourages Draco to quickly kill Dumbledore, and blasts Greyback aside when Greyback tries to attack Dumbledore. Right after Snape kills Dumbledore, Harry hits Yaxley with the Full Body-Bind (HBP28). The Ministry found him, still Petrified, in the Tower (HBP30). Presumably he was taken to Azkaban, but he was free by the next month.
Yaxley was in charge of Voldemort’s takeover of the Ministry; he cast the Imperius Curse on Pius Thicknesse and was the one reporting to Voldemort about the Ministry’s doings (DH1). Despite this, he is not too high up in the Death Eater ranks; Voldemort makes him sit in the middle of the table during their meeting.
When Thicknesse became Minister of Magic after the murder of Scrimgeour, Yaxley became the new Head of the Department of Magical Law Enforcement. In this capacity, he was also part of the Muggle-Born Registration Commission. He used his position to bully others by threatening to impugn the Blood Status of their loved ones. Among such victims was Reg Cattermole, who was responsible for stopping the rain in Yaxley’s office (DH12). Yaxley’s interrogation of Mary Cattermole is interrupted by a Stunning Spell from Harry (DH13), but Yaxley ruins the Trio’s clean getaway by grabbing onto Hermione’s arm as she Disapparated from the Ministry to Grimmauld Place (DH14). This makes Yaxley privy to the secret of Grimmauld Place’s location, and eliminates it as a viable sanctuary for the Order.
During the Battle of Hogwarts, Yaxley dueled Flitwick just before Voldemort called a ceasefire (DH32). Both Yaxley and Flitwick survived the encounter and continued to participate in the battle. Towards the end of the battle, Yaxley was slammed to the floor by George Weasley and Lee Jordan (DH36); it is unknown if he survived.
Yaxley must be very adept at casting the Imperius Curse. He cast it on Pius Thicknesse in July 1997; Thicknesse showed no signs of regaining control ten months later at the Battle of Hogwarts, even though the victims eventually begin to fight back against the Imperius Curse.
There was some confusion over whether Yaxley is in fact the "brutal-faced Death Eater" from the Battle of the Tower. When Harry runs into him at the Ministry, it is said, "One of the Death Eaters who had witnessed Dumbledore's death was striding toward them." (DH12) Through process of elimination, this has to be the brutal-faced Death Eater. Significantly, Yaxley is described as having a "brutish face," (DH13) and such repetition is usually intentional on Rowling's part.
Although Harry cast "Petrificus Totalus" on Yaxley in the Battle of the Tower (HBP28), Scrimgeour claims to have found a "Stupefied" Death Eater at the Tower (HBP30). This led to the formation of several theories prior to the release of Deathly Hallows about a switch of bodies. That never panned out, and can either be explained as an error on Rowling's part, or as Ministry incompetence (being unable to tell the difference between Stunning and the Full Body-Bind).