The mercenary fleet begins almost by accident. While visiting his grandmother on Beta Colony, Miles acts on a sudden impulse by buying an outdated interstellar cargo freighter and employing its down-and-out pilot, Arde Mayhew. He offers the freighter's services to the beleaguered government of Tau Verde IV, a planet wracked by civil war. The government's opponents in the war have hired a mercenary fleet, the Oseran Mercenaries, to blockade the planet. By accident, Miles leads the Tau Verde representative to believe that he himself heads a mercenary fleet. Miles takes his freighter to Tau Verde and, through clever tactics and considerable luck, successfully captures a vessel of the Oseran mercenary fleet. Despite his vastly inferior force, he outmanouevres the Oserans at every turn; eventually the defeated Admiral Oser agrees to join Miles' fictitious fleet, giving the previously nonexistent 'Dendarii Mercenaries' (named for a mountain range on Miles' planet of Barrayar) a real existence.
Miles is summoned to Barrayar (see The Warrior's Apprentice in Vorkosigan Saga) to answer charges of treason. In clearing his name, he persuades the Emperor Gregor Vorbarra to declare the mercenaries part of the Barrayaran Imperial Military Service.
Miles's one-time lover, at the end of Memory she commanded the Dendarii as Admiral Quinn. She is a native of Kline Station and hates living on planets, which she luckily doesn't have to worry about, having declined numerous marriage proposals from Miles and also being a mercenary fleet Admiral.
Elli Quinn began the series rather unremarkable in appearance, but was horribly injured by a plasma arc, which scorched off her facial tissues and almost all of her features. Miles subsequently purchased her the finest reconstructive surgery money could buy, more than restoring her appearance, and she is now stunningly beautiful.
This Betan hermaphrodite became the Captain of the Ariel as a result of Miles's conquest. Thorne is fired from the Dendarii at the end of Mirror Dance for taking part in Mark's scheme to free the clone-slaves from Bharaputra's laboratories. Bel becomes a roving ImpSec agent and eventually settles in Quaddiespace, resuming a relationship with the Quaddie musician Nicol, whom Thorne had first met on a Dendarii mission to Jackson's Whole.
Sergeant Taura of the Dendarii Mercenaries started life as genetics experiment. She was developed as a super soldier on the planet of Jackson's Whole. She was the only eventual survivor of the experiment, that seems to have used some animal genes spliced into the human genome. In particular, she is "equipped" with fangs and claws. Taura suffers from a massively accelerated metabolism, giving her extremely fast reflexes and responses and very great strength, compounded with her 8-foot (2.4 m) height and matching mass. Unfortunately this accelerated metabolism will also sharply reduce her lifetime - something she is very much aware of. All attempts to find a medical way around this situation have (so far) had only marginal success. The last time she was seen in the series, in the novella Winterfair Gifts, she states that the doctors give her one more year to live, but also that they've said the same thing for the last four years. She is not concerned about it, deciding to live each day for its own sake.
Admiral Naismith of the Dendarii was supposed to kill her to save her suffering at the hands of her new owners and save her a painful death when her metabolism failed, however when he realised that she was a human being despite her looks, he switched his mission to rescue her, a "damsel in distress" instead. She then joined the Mercenaries as a (very successful) commando Squad Leader.
She dies of old age at the age of 30 at the Durona Clinic on Escobar. Despite pleas from Miles, Roic, Rowan, and Mark, she refuses to have herself frozen in cryostasis until a cure could be found for her condition. Miles named one of his daughters after her.
Instead of cryostasis, she was cremated, choosing "fire over ice." Her one request was that her ashes not be scattered on Jackson's Whole. Miles and Roic buried her urn on the family plot on Barrayar.
Miles Vorkosigan found Baz living in a recycling depot on Beta Colony, after Baz deserted from the Barrayaran military for reasons unknown. As his desertion was technically in the heat of battle, his penalty would be quartering ("cut into four pieces", not "domiciled"). Because of his engineering expertise he was recruited to join Miles on his first mission, to Tau Verde IV and thereafter made a place for himself in the Dendarii Mercenaries as Fleet Engineer. He married Elena Bothari and they had a baby daughter, Cordelia.
Ky Tung is a military history expert from Brazil, Earth. He commanded the Triumph prior to its capture during the Tau Verde war, by the Dendarii. After being mishandled by his former commander, he switched his loyalties to the Dendarii and became Chief of Staff.
Pilot Officer Arde Mayhew is a cargo ship jump pilot, whose neural implants, necessary to guide a ship through a wormhole jump, are for an obsolete class of jump ships. He has been medically disqualified from receiving new implants.
Arde is first seen in Shards of Honor when Captain Cordelia Naismith cons him into giving her an off the record lift from Beta Colony to Escobar. Miles encounters him approximately 18 years later, when he comes across the down-on-his-luck Mayhew attempting to fend the wreckers off from the last known ship which he is able to pilot.
Miles buys the ship and hires Arde as its pilot, but the ship's jump engines are irreparably damaged in Tau Verde when Arde uses it to ram Ky Tung's Triumph, facilitating its capture.
Arde stays on with the Dendarii as a shuttle pilot.