Dragoon Form

For a Dragoon Spirit to be created, a Dragon first must die. Dragoons are Dragon Knights, an existence that surpasses Humans as they channel the spirit – the essence – of the deceased Dragon within their Dragoon Spirit. They are capable of soaring through the sky and casting magic. Though Dragoons in the past ruled over Dragons, those ancient creatures are now regarded as elements of legends.

Do you know what is the source of the power of Dragoons? War is not in human nature. Humans fight by making themselves enter insanity. It is... the same for revenge. This is a tool to amplify the insanity. And this insanity is the source of the power of Dragoons, the Dragon Knight. Rose, Disc 2: The Queen Fury

Transforming into a Dragoon outside of battle is difficult, and it can be assumed that being a Dragoon requires strength of spirit so as not to be taken over by the insanity that comes with the power.

Once a party member has acquired their Dragoon Spirit, they can transform upon accumulating at least 100 Spirit Points in battle. SP carries over from battle to battle and is gained when executing an Addition (failing the Addition, but still managing to hit some triggers gives you a fraction of the set SP), though the item Spirit Potion also instantly gives you a set amount of SP. SP gain can be enhanced by wearing SP-boosting equipment. A character's Dragoon Level increases as they accumulate SP throughout the game, capping at 5. As their Dragoon Level increases, so does the amount of maximum MP (MP is not used outside of Dragoon form), the amount of SP that can be stocked up for battle (capped at 300), the available magic spells and stats increases.

With higher SP, spending consecutive turns in Dragoon form is possible (each of them costing 100 SP), which is useful due to abnormal status immunity and each Dragoon having an elemental affinity that also protects them from the respective element. However, Dragoons cannot use items, defend in order to regain HP and reduce damage, or revert their Dragoon transformation as long as SP remains. This is a drawback especially in boss battles that feature enemies who have an advantage over Dragoons or have an elemental advantage while you require a particular support spell that only Dragoons can cast. (Yes, this is bad game design.)

In Dragoon form, Additions change in input method (though they, too, can end prematurely) and in the fact that they remain the same throughout the game unlike the regular Additions, which can be set through the menu. D-Additions add to respective Dragoon's element to the physical attack. Although powerful in the early game, their usefulness drastically declines in the later part of the game when regular Additions are far stronger.

Rose already has been recognized by a Dragoon Spirit by the time she joins the party (rather than inheriting it like Dart or obtaining it from a story boss as in most cases) and is the only character capable of transforming into a Dragoon from the start.

A video of Rose's Dragoon transformation can be found here.

Dark Dragoon Spirit

Rose's D-Additions are dark-elemental, as is her Dragoon magic. The Dark Dragoon Spirit is special in that it is neither primarily about offense nor support; its spells may not deal much damage, but they are the only ones capable of triggering a secondary effect. Of the seven Dragoons, four know at least one utility spell of some kind (a combination of healing, damage resistance, reviving and status ailment recovery), with Rose's utility lying in her first and thus initially available spell. Unlike the other support spells, Astral Drain's utility is only part of the spell, as its HP Drain effect is based on the amount of damage the attack inflicts on the enemy. Rose is also the only Dragoon capable of casting instant death or an abnormal status effect (Fear, which halves attack and defense of enemies).

Once a character has reached the last Dragoon Level (5), they learn their final spell, which enables them to summon a Dragon. It is assumed that the Dragon summoned during these attack sequences is not a living being, but the spirit – the essence, the soul – of the Dragon that resides within the respective Dragoon Spirit. After all, a Dragon has to die for a Dragoon Spirit to be created, and Dragoons in the past were said to be the incarnations of Dragons themselves – figuratively so, since their powers are derived from the Dragons that died. These Dragons are not to be confused with Vassal Dragons, which Dragoons commanded in battle. This would also explain why the Dragons from these attack sequences aren't featured anywhere in the game outside of battles.

A video of Rose's Dragoon attacks can be found here. Yes, unlike the rest of the group's spell animations, they are quite disturbing - you should see the original version of Demon's Gate (originally Hell's Gate).

Spell Effect MP Learned
Astral Drain Dark STR 25% Single & HP Recovery 10 -
Death Dimension Dark STR 25% All & Fear 20 D.Level 2
Demon's Gate Lethal Attack for all 30 D.Level 2
Dark Dragon Dark STR 100% Single 80 D.Level 5