The station of Protector of the Universe was created shortly after the creation of the universe. Eon, the offspring of Eternity was charged with protecting the Celestial Axis, Time; nurturing the evolution of sentient life of the universe. To this end Eon saw the need for the universe to have a protector. He created the Quantum Bands, devices of nearly unmatched in power to act not only as tools for his protector but as symbols of the station. A Protector required the necessary attributes of Intelligence, Courage, and Resourcefulness to be a worthy of the Quantum Bands and Eon’s universal protector. In many cases it is believed that Eon will appoint a Protector for a particular mission, not only to protect the universe, but to protect it (or a part of it) from a particular threat such as, Thanos or Maelstrom. Only one appointed protector should be operating at a time as appointed by Eon or her child, Epoch. For nearly as long as the universe has been in existance there has been a Protector.
It's worth noting that there is a vague connection between the Protectors of the Universe and Cosmic Awareness. Eon being a time being and an abstract entity is sometimes known as the Keeper of Cosmic Awareness. As such Eon has been known to bestow Cosmic Awareness upon his Protectors, namely Mar-Vell and Wendell Vaughn. In the case of Mar-Vell the Awareness was even passed down to two of his children Genis-Vell and Phyla-Vell to some degree.
Over the course of millenia, there have been presumably countless beings appointed by Eon to preotect the universe, but only a few are known to us.
The first Protector was known to be Glakandar the Stygian Starbender who operated early in the universe’s history most of his history and the story of his life has not be revealed.
Another early Protector was Ree of the Archeopians, a bird-like species that was among the first to have their homeworld consumed by Galactus. As such it is assumed that Ree was among the very first Protectors, it is unknown if he died in defense of his homeworld or not. Much of his life is unrevealed.
A later Protector was Trantra of the Trill, who was the first non-humanoid Protector of the Universe. Much like the others, most of his life has been unrevealed.
In recent times Eon appointed the Kree soldier Mar-Vell to defeat the star-killer Thanos. Though he never wore the Quantum Bands, he succeeded in defeating Thanos. He later died due to the ravages of diesease. Mar-Vell is perhaps the best known Protector of the Universe in recent times.
Mar-Vell was succeeded by Wendell Vaughn, an earthman who had donned the Quantum Bands years before. Calling himself Marvel Boy, then Marvel-Man, then finally Quasar and acting as security chief at Project PEGASUS. He was charged with protecting Eon himself from the Cosmic Assassin, Quasar failed in this; being forced to kill Eon himself in order to protect the secret of Cosmic Awareness, but he did plunge the Cosmic Assassin in to a black hole destroying him and literally saving the universe. Quasar was later slain by Annihilus.
The Quantum Bands were later taken from Annihilus by Phyla-Vell (daughter of Mar-Vell) though she had never been an appointed Protector.
After Eon’s death his station was taken up by its offspring Epoch and after Vaughn’s death appointed Greg Willis and augmented his power to protect her from Galactus. After successfully defending Epoch against Galactus, Willis declined to formally take the station of Protector of the Universe.
As of the Annihilation War, there has not been an active Protector of the Universe with the death of the Wendell Vaughn, although Greg Willis briefly accepted the role after being appointed by Epoch and for a time after Annihilation Phyla-Vell considered herself the unofficial Protector with her wielding the Quantum Bands, perhaps she was simply ignorant of Epoch's role in appointing the Protector of the Universe. Quasar is still considered the Protector of the Universe with his recent resurrection according to Phyla-Vell and Nova Prime, Richard Rider. Whether or not this is truly the case, only time will tell.
Kree soldier Noh-Varr was appointed "Protector of the Universe" by the Kree Supreme Intelligence, obviously a case of confused continuity.