Geiser is the AI or the Biological O/S in charge of the Terraformer Ark.
The Terraformer Ark was meant to be the 999th of the 1000 ships built by the humans of Marble Blue to escape the threat of The Entity. However, the Ark never managed to leave the planet: it crashed, killing all the humans on board. Geiser survived, but his life support system was broken.
The Entity turns out to have come to Marble Blue not to destroy it, but only to lay its egg on it. Since his life support system no longer works Geiser had to take control of the Zodiac Arm Sagittarius and used it as his avatar in the outside world. Every 100 years, he would come to the World Tree, harvest a fruit, and use it to sustain himself.
A millennium after the Entity's departure, during another visit to the World Tree Geiser encounters the Protagonist, his friends Daisuke and Leary and the criminal Shyamalan, who are all seeking a World Tree fruit themselves, knowing it has the ability to cure all illness. Geiser explains to the humans that the fruit only works on non-humans. Before leaving, Geiser takes the fruit for himself and tests the Protagonist's team in a fight.
Later, when Mariv under the Geofront Elevator comes under assault by the Incarnatus Axiom, which has been unleashed into Theia by Volhelm and defeated by the Protagonist, Al and Johannes beforehand, Geiser appears and kills Axiom, then just as quickly leaves.
When the Protagonist's team is sent to explore the newly-discovered Terraformer Ark, they encounter Auz, who sends Volhelm Soldiers after them. They quickly get rid of the soldiers, prompting Auz to fight, but before he could do so Geiser interrupts him by separating him from the team.
In the council chamber of the Ark, the Protagonist's team find Geiser in his true form. Geiser introduces himself and explains the whole story to the team before testing them again, in the form of Sagittarius. Just after the fight, Auz suddenly enters the chamber and breaks Geiser's hardware, seizing control of Sagittarius from him. Before he dies, Geiser gives the World Tree fruit to the team and telling them that ever since he met them, he knew he wouldn't need to live for another century.
After the events of his death, Chancellor Steiner gave the Protagonist and his team a mission to turn off the failing life system support so that Geiser can finally rest in peace. They did so by uninstalling the Geiser O/S, thus laying Geiser to rest and Theia thanked them for their help.