The Guardian also known as the Colossus, is the main antagonist of the Islands story arc of Adventure Time.
It is a monolithic and immensely powerful robot built by the Founders to protect humans from the outside world after the Mushroom War. However, it becomes more of a force of control then safety, trapping all the humans on the Islands and stopping anyone from getting in or out.
The Guardian first appeared in a flashback told by Martin the "The Visitor" in which it is depicted as a monstrous, faceless entity with glowing bright eyes that separates Martin from Finn in the middle of the ocean.
In "Whipple the Happy Dragon", the second part of the Islands miniseries, the Guardian reappears and intervenes in Finn, Jake, Susan and BMO's voyage by crashing their boat, which was on rout to the Islands.
In "Hide and Seek", the fifth part of the miniseries, the Guardian is revealed, through a song by Dr Gross, to have been built many years ago by the Founders (the original settlers of the Island). The robots main purpose was to protect humans from the dangers of the outside world.
In "Min and Marty" it is revealed that the Guardian had mistaken Martin (who was stranded at sea trying to save Finn) as attempting to flee the Island and tries to kill him by slashing its gigantic arm into the raft containing Martin and Finn. This creates a massive eruption of water which propels Finn and the raft into open water whilst Martin jumps onto the Guardians arm and shoots a laser beam into one of its eyes, exploding it.
In the final part of the Islands miniseries, Finn, Jake, Susan and BMO, attempt to leave the Island once and for all, however the Guardian reemerges and attempts to destroy their ship. It is then revealed that this was merely a hologram created by Frieda and Susan in order to distract the robotic monster whilst Jake wraps his body around it in order to allow Finn to dive into its one destroyed eye. Finn delves deeper into the machine with a cable in hand, whilst Jake attempts to fend it off. Finn eventually finds the Guardians main reactor and plugs the cable into a socket, allowing Finns mother, Minerva, to upload her AI and deactivate the Guardian. The Guardian then sinks into the water, allowing Finn and the others to escape, whilst the rest of the Islands human inhabitants are now given the choice to leave.