|The following information is from the Adventure Time comic book series. It may not be part of the same canon or continuity as the animated series.|
Chronologius Rex is a character presented in the 1st issue of the spin-off comic "Beggining of the End". They are the embodiment of time, living in the center of Time, the Vanishing Point, or also known as the Forth Dimension.
All-Time has the power to create portals in time, and delete people from time. It first appeared because Finn broke an Oath in All-Time's name. All-Time following rules given to them by some unknown source, plans to erase Finn from time because of him breaking the Oath "Jake I promise, when we grow up, we'll live here, and be best friends, and spend every day together, having adventures for all time". They then allow Finn to say goodbye to Jake and Princess Bubblegum, then All-Time lets Finn speak to the Time-Echo versions of Shoko, Fern, Susan Strong, Canyon, and Billy. All-Time later allows Finn to see a couple of his possible futures, one where Finn is the Champion of the Candy Kingdom for the rest of his life, another where Finn becomes an adventurer teacher on Founders' Island, and finally, one where Finn is a space captain. In the end Jake comes to the Forth Dimension to save Finn, and All-Time has a change of heart and decides to let Finn and Jake escape the Vanishing Point.
He is a large being with six eyes, though he says he has infinite eyes. His nose is shaped as the infinity symbol. He has at least eleven arms, and what appears to be a beard. He has the general shape of a caterpillar. All-Time seems to not care about how Finn feels and how it may affect things. However, once Jake comes to the Forth Dimension] to save Finn, All-Time didn't see this as possible, so he decides to allow them to escape.