Quaros is the seventh guardian, once a part of a group with the six new guardians that have taken the mantle of Guardians of the Universe, after Sinestro and Atrocitus killed the previous guardians.
Rise of the Third Army
For eons, Quaros and the other guardians were hidden and sealed as wardens in the Chamber of Shadows, along with Volthoom, the First Lantern, who they were assigned to oversee. When Quaros and the other guardians finally come face-to-face with their estranged siblings, they are shocked to see how emotionless the latter have become and briefly provide resistance, which results in one of Quaros' brothers being killed. They are again locked into the Chamber of Shadows until finally Simon Baz releases them.
Later, after Volthoom and the Guardians (save for Ganthet and Sayd) are killed, Quaros and the other guardians step into the role of Guardians of the Universe, though they relinquish their responsibility to Hal Jordan in order to experience the universe they had been earlier denied.
After spending some time learning how the universe has changed, Quaros' siblings are ready to finally accept their roles as Guardians of the Universe, and begin assisting Kyle Rayner in his quest to help those who need it.
Quaros, though, feels as if he has more to learn from studying the universe. He alone leaves the group to continue his studies.