The In-Betweener is a cosmic entity who exists as the synthesis of the major concepts of the universe: life and death, reality and illusion, good and evil, logic and emotion, existence and nothingness, god and man. Variable in stature, the In-Betweener is an agent of Lord Chaos and Master Order, two of the universe's principal abstract beings, whose appointed task is to maintain the universal balance. Simultaneously existing and not-existing, present everywhere and present nowhere, the In-Betweener is sometimes considered the opposite of Galactus, the being who exists between the principalities of Eternity and Death. While the In-Betweener is the embodiment of opposites, Galactus is their absence (neither good nor evil, servant of life or death, etc.).
The In-Betweener first came to human attention when it abducted the golden humanoid Adam Warlock to its quasi-realm, and attempted to transform him into a champion of life to help restore the balance upset by Thanos of Titan. The experience drove Warlock insane, creating the entity known as Magus.
The In-Betweener was later encountered by Earth's sorcerer supreme, Doctor Strange, when it empowered the spells of a renegade band of wizards calling themselves the Creators. Strange opposed the In-Betweener and its plan to restore balance by creating universal insanity, and calling upon its masters, Order and Chaos, Strange managed to halt the In-Betweener's machinations.
On one occasion, the In-Betweener exceeded his mandate by attempting to overthrow and destroy Galactus, and his place in the cosmic hierarchy. With the assistance of the Elders of the Universe, he tried to gain control of the Soul Gems for his plans. The In-Betweener was defeated in part thanks to the Silver Surfer, Reed and Sue Richards, and turned over to Lord Chaos and Master Order for punishment. The In-Betweener was imprisoned at the apex of order and chaos in order to neutralize his powers, although his patrons allowed him use of his abilities inside his crystal cage as a gesture. In-Betweener retained the Soul Gem from his previous adventure, making him a target for Thanos. The Mad Titan manipulated the In-Betweener into breaking out of his prison, drawing the ire of Order and Chaos while Thanos made off with his Soul Gem.
Although the In-Betweener is cosmic in scale, aspects of his being have occasionally taken mortal form and interacted more directly with humanity.
The In-Betweener possesses sufficient power to alter reality on a cosmic scale. However, the In-Betweener is not all-knowing or infallible. Indeed, within the parameters of the In-Betweener's existence are both power and weakness, knowledge and ignorance.