Five Pebbles (character)

Five Pebbles is the iterator met at the heart of the Five Pebbles supercomputer. He is one of the two such beings you can meet in the game, the other being an ailing Big Sister Moon. Five Pebbles is non-aggressive to the slugcat. He gives you a special mark that floats above the Slugcat's head that lets you understand him and, once he can talk to you, gives you directions to escape the facility and ultimately complete the game.

His dialogue is precise and clinical. He hints at feelings of dissatisfaction and the futility of his purpose (calling himself a "reluctant" gift to the world, for example). After talking to Moon later, it is revealed that the Rain started because of him using massive amounts of water to power himself, ejecting tons of water vapor into the air. He also created the Region Garbage Wastes by dropping massive amounts of waste and sludge to the ground. His mental state is unstable, according to Moon, who, by the way, was nearly killed by Pebbles using four times the amount of water he needed and draining Moon of nearly all her water. An Ancient found later tells the slugcat that the mark is an incredible gift.

Pebbles' Insanity (Spoilers)
Pebbles was one of seven known iterators, working to solve the Great Problem. The problem of ascending without the Void Fluid was so great to the ancient civilization that ruled the world that they created huge, complex supercomputers the size of large buildings. The artificial intelligence within worked to figure out this Great Problem. Pebbles is one of them. After Sliver of Straw sent out a message to alert the others a solution had been found, they immediately died and their research was lost. This is what drove Five Pebbles to madness, using up far more water as his systems would overheat from the heavy research he was doing. The excess water vapor formed massive clouds that started the Rain. The ancients were forced underground by this, but left the iterators on to keep working. He also dumped huge amounts of leftover waste, forming the Garbage Wastes region. It is this state of calm insanity the slugcat finds him in thousands of years later.

A fair bit of warning, if you visit him for a second time, he won't be so happy to see you but will let you stay for a little bit. Stay too long and he will slam you into the ground, killing the slugcat. On third visit, he becomes hostile and warns you to get out immediately. If you do not heed his words, he will proceed to slam the slugcat onto the ground in a few seconds.

Backstory (spoilers)
Five Pebbles is one of many semi-organic supercomputers known as iterators. The other iterator you can encounter is Moon.

Completing Moon's optional side quest after encountering Pebbles reveals that Five Pebbles is corrupt and is the cause of the Looks to the Moon facility's collapse. In addition to this, Five Pebbles is also responsible for the deadly monsoons due to the amount of water he uses to cool his systems.

Pebbles' dialogue with you hints at the fact that he was left here by a civilisation that no longer exists. The lore Moon can read for you confirms this, as well as naming other iterators who may or may not still be active, and revealing the reason they were all built.

Trivia

 * Five Pebbles can move around his chamber while Big Sister Moon can't, because of the damage she has sustained.
 * He has a cloak... yes!
 * Five Pebbles reveals that his hardware is succumbing to damage -- and by extension the other remaining iterators are probably in similar states, apart from Moon, who is worse off.
 * Five Pebbles has no HP in his code, rendering him as immortal/unkillable.
 * He is most likely the cause behind the Daddylonglegs and the Brotherlonglegs existing

Dialogue (spoilers)
A little animal, on the floor of my chamber. I think I know what you are looking for.

You're stuck in a cycle, a repeating pattern. You want a way out.

Know that this does not make you special - every living thing shares that same frustration. From the microbes in the processing strata to me, who am, if you excuse me, godlike in comparison.

The good news first. In a way, I am what you are searching for. [I] and my kind have as our purpose to solve that very oscillating claustrophobia in the chests of you and countless others. A strange charity - you the unknowing recipient, I the reluctant gift. The noble benefactors? Gone.

The bad news is that no definitive solution has been found. And every moment the equipment erodes to a new state of decay. I can't help you collectively, or individually. I can't even help myself.

For you, though, there is another way. The old path. Go to the west past the Farm Arrays, and then down into the earth where the land fissures, as deep as you can reach, where the ancients built their temples and danced their silly rituals. The mark I gave you will let you through.

Not that it solves anyone's problem but yours.

At the end of time none of this will matter, I suppose, but it would be nice if you took another way out. One free of... frolicking in my memory arrays. There is a perfectly good access shaft right here.

(If you continue to stay:)

That is all. You'll have to leave now.

(5 more seconds:)

LEAVE.

(Left after this, presumably he slams you into the ground)