Explanation of God’s Right Seat and the Archangel Attribute

Acqua of the Back is blessed with the "Power of God" attribute, as he is aligned with the Archangel Gabriel after purifying his body of the Original Sin. As a result of his alignment with Gabriel, he is associated with the color blue, the water and the moon. He wears a sporty polo over a long sleeve shirt with blue slacks and holds the position of the "back" as his name indicates.

A man stood a short distance away. “Who are you!?” shouted Kamijou hostilely. The man had Vento’s limp form held under one arm. He wore a blue long-sleeved shirt with a white short-sleeved shirt over it. For pants, he wore thin slacks that looked like they had decent breathability. It was a sporty outfit, but it lacked energy. It reminded Kamijou of the golf wear men in their prime liked. While holding a chic black umbrella, the man was surrounded by a silent and solid aura that a high school boy like Kamijou could never hope for. But what was the most frightening was how the man showed no tension at all despite the scene of destruction before him. His white skin and his brown hair both looked like sharp blades. “My apologies,” said the man in fluent Japanese. “I have business with this girl. I blinded you to keep you from growing violent, but perhaps that did not sit right with you.” “I asked who you are!” “I am Acqua of the Back. I am a member of God’s Right Seat just like Vento.”
(Toaru Majutsu no Index Volume 13, pg 219-220)

“Vento of the Front, Terra of the Left, Acqua of the Back. If they each represented the four archangels, then Acqua represented Gabriel, his element was water, and during the last battle, he hadn’t used any water spells because with the abilities of the fighters there, there had been no need to use them.”
(Toaru Majutsu no Index Volume 16, pg 91)