Gadra

Gadra was introduced as the Imperial Court Magician of the Eastern Empire and is the master of Razen. He has lived for more than a thousand years -- every time his life was about to come to an end, he would undergo the secret ritual he invented—the mystic art of ‘Reincarnation’—in order to rewind his lifespan.

Appearance
Gadra has the appearance of an old man with very long white hair and beard.

Personality
He has been described by Rimuru as a born egotist and that he has no relation to things like loyalty. However, he appears to value people close to him a lot, so much that avenging Adalman's death more than a thousand years ago is the sole reason that he allied himself with the Eastern Empire.

He is also an opportunist that's quick to adapt to changes. When he learned that the Clerics of the Seven Luminaries (his subject of revenge) were eliminated, and Tempest turns out to be a much greater threat than expected, he defected to Tempest without hesitation.

Finally, his another defining trait is his fascination in magic. Every time he reincarnated, he would read the treasured books of each royal palace library and build up his knowledge. He held grudges against the Eastern Empire for phasing out his Magic Corps in favor of the Armored Corps.

Background
Gadra was a close friend of Adalman when he was alive. As Adalman was killed due to nefarious schemes concocted by the Seven Luminaries, Gadra performed his Reincarnation magic in an attempt to revive him. However, because the ambient miasma had tainted his soul, Adalman was reborn as a wight. As a result, Gadra swore revenge against Luminism and the Seven Luminaries, and allied himself with the Eastern Empire.

Throughout his few hundreds years of service in the Eastern Empire, Gadra had been ushering the Empire to wage war against the west. He is one of the survivors of the previous Veldora subjugation operation.

He defected to Tempest alongside Shinji and his team, and has been given the guest treatment since. Gadra ended up as the new guardian of the 60th floor and became a hidden boss besides the Demon Colossus.

Trivia

 * He is a Reincarnator, but rather than being a human who died in another world and reincarnated in the current one, he is a native to his world and has instead reincarnated numerous times in order to extend his lifespan using magic.