Born at the beginning of the universe, with the first act of willpower, Ion is a cosmic entity. One the seven entities personifying the Emotional Spectrum, it is the living embodiment of the willpower in the universe. It is a passive creature, living in symbiosis with it's host as opposed to the parasitic relationship some of the other Emotional Embodiments employ.
When the Guardians of the Universe were made aware of the Emotional Spectrum, they would be drawn to the willpower found at it's center. Seeking to put an end to the destruction caused by Parallax, the Guardians captured the fear entity. They also captured Ion, knowing that it possessed enough power to change the course of the very universe itself.
Both were placed in the Central Power Battery on Oa, while Parallax would become responsible for the yellow impurity in the rings, Ion only lent them power. There it would stay for untold millennium, until the Guardians decided to incarcerate the rogue Green Lantern Sinestro in the Battery, as well. In the course of his plan to set free Parallax, he also freed Ion as well, thought it's potential would remain untapped until much later.
The first known host of the Ion entity was Kyle Rayner, following the death of Kyle's girlfriend at the hands of Major Force. Kyle then destroys the hate within him and releases the villain Oblivion, who Kyle defeats. Then result of this is that when he left the sun to search for Hal's body, the energy he absorbed attracted the Ion entity to him, and he was also bestowed it's powers, as well as tapping into the Corps' powers. Kyle then takes the identity of Ion. He then revives the deceased Guardians and recharges the Central Power Battery. Kyle then loses his Ion powers after restoring Jade's powers.
Ion Once More
Kyle then retrieves his Ion powers after Jade's death during the Rann/Thanagar War. Upon her death, she gifted him with the remainder of her power, allowing the Guardians to place the entity in him once more. Kyle accepts the role of Torch Bearer for the Green Lantern Corps. Kyle then leaves Earth to find his future in space after a series of events.
Sinestro Corps War
The existence of the Ion entity wasn't revealed until the Sinestro Corps war. Captured on Qward, Kyle was surprised when Sinestro stripped him of the entity. Hoping to use it to better understand life, the Anti-Monitor attempted to study it by running a succession of tests. The Lost Lanterns would arrive to free Ion, though it cost them their leader Ke'hann in the process. Gathering the rest of their forces, they returned to Oa with the Ion entity in tow.
A New Host
Upon reaching Oa, Ion was returned to the Guardians. Though they wanted more time to prepare themselves, the assault on the Multiverse by the Anti-Monitor's Heralds forced their hand. They looked through their Corps for suitable candidates, feeling that Kyle Rayner had become tainted by his time in Parallax's thrall.
Though they considered others, such as Arisia, they settled on young Daxamite Lantern, Sodam Yat, who was prophesied to be the "Greatest of All Green Lanterns." Teleporting to Earth with Ion in tow, the Guardians stopped the Anti-Monitor from attacking, then merged Yat with the Ion entity, infusing him with power.
After the Sinestro Corps War, Daxam was taken over by the tyrant Mongul. While trying to free Daxam from Mongul's grip, Sodam was trying to liberate his race from slavery on the Sinestro Corps. Sodam decided he had to battle Mongul, but was denied the power to tap into Ion by the corrupt guardian, Scar. As a last desperate act, Sodam decided he had to take his ring off so he can access the ion power, and fly into Daxam's sun, turning it yellow and empowering his race enough to rebel against Mongul and his legion.
Now being trapped in Daxam's sun, Sodam Yat was visited by the same enemy that kidnapped parallax. The Enemy reaped Sodam of Ion, which caused the sun to turn red again and for all of the Daxamites to lose their powers. Now the enemy has six entities that he seeks.
In "Green Lantern: The Animated Series" it is revealed that the Guardian Scar used a portion of Ion's sentient energy to give life to the A.I. Aya