Morgan was the Council's "guard dog" and Dresden's probation officer of sorts, assigned to make sure he didn't break any of the Laws of Magic, usually coming around when Dresden cast a spell. He has consistently gone out of his way to instigate him. Dresden, following an encounter where Morgan was willing to die in order to bring down someone he believed was a traitor, realised that Morgan was a good guy, but, like a worn-out cop, he had seen too much.
Donald Morgan carried a Warden's sword, symbol of the authority given to them by the White Council. These swords, created by Captain Luccio, were imbued with magical energy to add metaphysical mass to the sword strokes of the wielder, and could "cut through" any magical construct or enchantment they came in contact with, making them an incredibly potent weapons against magic-wielders.
Morgan was a stickler for rules and for technicalities, and was particularly loyal to the Laws of Magic. He did not question the White Council and its policies, and he was willing to sacrifice his life for it. As one of the most senior and devoted wardens, he also acted as the Council's executioner. He was the apprentice of Anastasia Luccio, for whom he had developed unreciprocated romantic feelings during his apprenticeship.
Morgan excelled in earth-based evocations, as he was seen using both pincers of earth and a shockwave created by stomping the ground in Dead Beat. He was an accomplished magical duelist, and Warden Luccio recognized him as being one of relatively few people qualified to take on Nicodemus Archleone in Harry Dresden's place. Morgan was even more capable in pitched battle, recognised as a war hero for his efforts in Proven Guilty, cutting down a Duke and two Counts of the Red Court who simply got between Morgan and the Red King. This led to a commendation from Queen Titania of the Summer Court, wherein he was inducted into the Order of the Silver Oak, a high honour of the Summer Court. Other examples of his accomplishments include wiping out forty zombies with minimal effort inside a minute, and luring a Naagloshii onto a government weapons testing site, leading it there to be destroyed by a nuclear weapon.
In the series
In Storm Front, Morgan corners Harry Dresden by Victor Sells' lake house accusing him of binding others to his will after Dresden gave Toot-Toot a job in exchange for food. "I remind you that you are under the Doom of Damocles. No further violations of the law will to be tolerated. Sentence to be carried out at once." Dresden reminded him that the Fourth Law of Magic applies to mortals, not Faeries and Toot had a choice to accept Dresden's deal. Dresden considered to himself that Donald had the ability to rip the hearts out of Tommy Tomm and Jennifer Stanton in the case he was working on for the cops. Morgan accuses Dresden of being the one who used magic to kill two people. When he later witnesses Dresden in at Victors' estate, he rescues him and testifies for the White Council. With him as witness, the Doom of Damocles was lifted off Dresden.
In Summer Knight, Morgan is present at the White Council hearing in Chicago. He forces Harry Dresden to remove the wrappings covering the injury caused by Mab, despite the wizard's protests. After the trial is settled, Morgan confronts Dresden at his apartment and unsuccessfully attempts to goad him into fighting him so the Warden can kill him at the Merlin's suggestion and bring and end to the war with the Red Court. Later in the story, when Dresden phones Edinburgh in an attempt to contact Ebenezar McCoy to get backup Morgan grabs the phone first and angrily hangs up on him after accusing him of being a traitor.
In Dead Beat, Morgan shows his distaste at Captain Luccio drafting Dresden into the ranks of the Wardens due to the losses of their members. When the the Corpsetaker steals Luccio's body, Dresden shoots and kills her. Morgan doesn't realize the situation and, thinking Dresden has murdered Luccio, attacks Dresden in a rage. At the last minute he is stopped by Luccio, now in the body that Corpsetaker vacated, who reveals with the help of Warden Ramirez that Dresden had killed Corpsetaker and not Luccio. After the battle Morgan admits that while he no longer believes Dresden's intentions are malicious, he insists that Harry Dresden lacks discipline and judgement and affirms that he doesn't like the Wizard before handing him his first paycheck.
In White Night, with Anastasia Luccio running her boot camp for warden trainees, Morgan is the de facto active field commander of the Wardens, and is in India to help the White Council's allies there.
In Turn Coat, Morgan is accused of treason when he's found over the body of Aleron LaFortier, and escapes from prison to seek sanctuary with Harry Dresden. Complications ensue when it transpires that Morgan has long been in love with Anastasia Luccio, who was then in a relationship with Dresden. Morgan is badly injured when escaping and becomes seriously ill while staying with Dresden, yet is still vigilant enough to critique Dresden's behavior, watch Molly Carpenter for signs of black magic, and pine after Luccio. Morgan turns himself in for the murder to save the person he believes guilty. Dresden reveals the true culprit and is almost killed, but Morgan intervenes, pushing his injured body past the breaking point. Morgan and Dresden make their peace moments before Morgan dies of his injuries.
In "Journal", Donald Morgan muses, as his last entry in his journal, about his relationship with Harry Dresden. It is revealed to promised Margaret LeFay that he would protect him, but later worried that Justin DuMorne had infected him with Nemesis, which might cause a Destroyer being birthed among humanity.