Dragoon Spirits are born from dying Dragons. Rose became the holder of the Dark Dragoon Spirit during the reign of the Winglies (but before the start of the Dragon Campaign), and the dead Darkness Dragon left behind a newborn. Rose named the baby the Black Burst Dragon Michael, who then fought at her side in war as her Vassal Dragon.
Rose: Darkness Dragon... Sorry. We don't have time. Winglies are mighty. Mighty enough to annihilate us, and the entire species. In order to liberate Humans, we have to sacrifice you, Dragon. Now is the time to decide, extinction or war.
Zieg: We choose "war". The Dragoon Spirit left by the Darkness Dragon, and this newborn dragon will be the spearhead of the war. Disc 4: Flashback
Like Rose, Michael survived the Dragon Campaign and continued to stay with her for more than 10,000 years. Michael is the strongest Dragon of all (not counting the Divine Dragon, the King of Dragons sealed away by the Winglies), boasting the highest attack, defense and agility, its shell a special armor that repels any attack, physical or magical.
Though a Dragon possesses enormous physical endurance, its heart eventually cracks under the weight of the many battles; when that happens, it forgets its Dragon pride, becomes savage and starts to attack Humans blindly until it no longer obeys the orders of its Dragoon master. After over 10,000 years, Rose was forced to kill her dear and loyal companion – perhaps the last emotional remainder of her past – with her own hands.