A Turtle of massive size with an ancient appearance, Maturin existed long before the creation of the mainstream universe in Stephen King's novels. By nature, Maturin was kind, wise, loving, gentle, compassionate, benevolent, and had a very grandfather-like demeanor when he spoke to humans. Maturin was said to be constantly withdrawn into his shell, sleeping and rarely ever coming out for anything. One of the occasions that Maturin did come out of his shell was when he had a stomachache which caused him to vomit out the mainstream universe, after which he promptly retreated back into his shell. Because of that, It (a creature that existed in the void within the Macroverse and was an adversary of Maturin, described by the latter as his "brother") considered the Turtle to be old, lazy, and stupid. However, given his vast amount of wisdom, power, and the aid, Maturin provided to those in need of it, It's perceptions of him seemed to be false.
In It, Maturin seemed to serve largely as a spectator to the events that unfolded in the battle between It and the Losers' Club. Maturin only involved himself and advised the young Bill Denbrough during the Ritual of Chüd in 1958, while Gan provided the Losers with most of their strength.
In the Novel
When the Losers faced It once again as adults 27 years later, It told Bill that Maturin was dead: he had choked to death on a galaxy or two. When the Ritual of Chud started again in 1985, Bill was able to see that It did not lie.
The name Maturin itself was a reference to Stephen Maturin, a naturalist from the Aubrey–Maturin series of novels, that were written by Patrick O'Brian, who discovered a new species of tortoise which was subsequently named for him.