True Dragon

True Dragons are the highest spiritual beings, and are considered the strongest existence in the world, far superior to both a True Demon Lord and a True Hero.

They are beings "perfect as one", existing as the most complete spiritual life forms. They have no attachment to their physical bodies, only taking those forms to respond to the desires of religions or similar (and also for convenience).

Birth
True Dragons are immortal and unblemished beings made of pure energy and formed by sentient beings’ prayers. They are basically god-like beings that represent and embody some aspect of nature.

After a True Dragon is born for the first time, it can never be truly killed. Even if the individual itself is killed, another True Dragon of the same natural characteristic will be born anew somewhere else. That new True Dragon will have the memories of the previous but will lose some aspects of its personality, while simply understanding its nature of existence right after birth.

Relationship
Although the True Dragons refer to each other as "brother" or "sister", they are not blood-related in the biological sense.

"Star King Dragon" Veldanava
The greatest of the True Dragons and the strongest among them. He is also the God that created the world in which they live. The husband of Lucia and father to Demon Lord Milim Nava.

"White Ice Dragon" Velzard
Partner of Guy Crimson, Velzard is known to be second in birth as well as in power from Veldanava. She is prideful and haughty, and was incredibly jealous when Veldanava granted Guy important roles instead of paying attention to her.

"Scorch Dragon" Velgrynd
Partner of Rudra Nam Ul Nasca, Velgrynd is the fastest Dragon in existence. Her very essence is that of 'acceleration' and 'high momentum'. Although slightly hot-headed, she is very compassionate, especially to her allies and family.

"Storm Dragon" Veldora
The youngest of the True Dragons. He is connected with Rimuru by an agreement and was known to frequently rampage here and there, therefore gaining the reputation of "the Evil Dragon" spread by Luminism. Known to act very childish.

Rimuru Tempest
During the Great War Arc Ciel made a new True Dragon subspecies for Rimuru by combining the properties of dragons, demons, slimes, celestials, & spirits (the original text is 竜魔粘性星神体). This new form mimicked the true dragons and made him stronger then even Milim's Battle Mode.

Milim Nava
A Dragonoid, she is one of the oldest and strongest Demon Lords. The daughter of Veldanava and Lucia, she inherited the greater part of her father's power resulting in many comparing her to True Dragons.

Gaia
As an offshoot of Veldanava's power and as his potential vessel for reincarnation, Gaia's body, and possibly Gaia himself, has the potential to reach the level of a True Dragon, as indicated by his corpse reaching the level of a Catastrophe class just from Milim's Demon Lord Awakening gift.

Charybdis
Similar to Gaia, Charybdis is an offshoot of a True Dragon, in this case an offshoot of Veldora, and inherits similar qualities, albeit being much weaker.

Dragons
An offshoot of a true dragon, to be more specific they are an offshoot of Gaia Milim's pet dragon. Once he was killed by an enemy nations the miasma of magicules from his corpse caused dragons to be created.

Trivia

 * In the first databook, Milim is mislabeled as one of the 4 True Dragons.
 * In the second databook, Gaia's race is listed as 竜種 which refers to True Dragons, without any further elaboration. It's unknown if this was an error or a deliberate hint of some sort.
 * Charybdis's Status Page in Light Novel Volume 3 refers to it as "Dragon" with the same kanji used for True Dragons 竜, unlike the kanji 龍, which refers to the regular dragons.