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 is kind and benevolent, having a very grandfather-like demeanor when speaking to humans. Maturin is said to be constantly withdrawn into his shell, sleeping and rarely ever coming out for anything.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 this, 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 he has provided to those in need of it, It's perceptions of the Turtle seem to be false.
In It, Maturin seems to serve largely as a spectator to the events that unfold in the battle between It and the Losers' Club. Maturin only involves himself to advise 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 face It once again as adults, It tells Bill that the Turtle is dead, that he had choked to death on a galaxy or two. When the ritual starts again in 1985, Bill was able to see that It did not lie.
The name Maturin itself is a reference to Stephen Maturin, a naturalist from the Aubrey–Maturin series of novels, written by Patrick O'Brian, who discovers a new species of tortoise which is subsequently named for him.