The character first appeared in Warlock #10 (Dec. 1974) and was created by Jim Starlin.
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He is later freed by Thanos, who secretly desires the Soul Gem the In-Betweener wears, as it is one of the six Infinity Gems.
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By the In-Betweener's own admission, he represents duality itself, specifically in respect to concepts such as good and evil; reason and emotion; truth and illusion; and life and death.
In-Betweener, a fictional character in comic books published by Marvel Comics
In Part One of the Thanos Quest, Thanos first travels to the Nexus of Reality where the concept being the In-Betweener is being imprisoned by Lord Chaos and Master Order; after freeing him from the sphere in which he is being incarcerated, Thanos forcibly takes the Soul Gem from the In-Betweener, whose powers are useless at the heart of the realm of Chaos and Order.