Vassal Dragon
A dragon who was directly subservant to its respective Dragoon was known as a vassal dragon. The two would fight together in battle with the dragon able to follow its own will as well as follow orders.
When a dragon was killed for its Dragoon Spirit, it was possible for a new dragon to be born and become a vassal. This was only depicted with Rose but it is possible all the original Dragoons had vassal dragons that simply did not survive the Dragon Campaign. Both Regole and Feyrbrand are dragons who fought alongside the carriers of their respective elemental Dragoon Spirit however there is no evidence that they were vassal dragons at that time or for Damia and Syuviel.
Battle
From the remains of the Darkness Dragon, a small dragon was born and immediately imprinted itself to Rose. She decided to accept it as her vassal dragon and gave it the name Black Burst Dragon Michael. Even though vassal dragons were subserviant, they were still wild and Rose has no misconceptions that Michael would be a tame beast.
Rose raised the dragon in the ways of war. When it came time to fight Michael had become one of the most powerful dragons, second only to the Divine Dragon itself.
Dragon Campaign
Michael accompanied Rose into battle and likely had an impressive kill count. During the battle of Kadessa, he easily dispatched Virage with both physical strength and his dreaded Dark Cannon. While Rose was the only Dragoon to survive the chaos, it appeared Michael was the only vassal dragon. This left the two to console one another after the deaths of their companions, specifically Rose's loss of Zieg.
Death
After so many years together, Michael was consumed by bloodlust and would no longer respond to Rose's commands. To avoid needless deaths, Rose was forced to kill Michael.
Having raised the dragon, she was the only one aware of its only weakpoint. After firing its Dark Cannon, Michael's heart was exposed. Using this to her advantage, Rose was able to kill one of the strongest beasts in Endiness and her own good friend.