Before the events of Kingdom Hearts, Scrooge McDuck traveled with King Mickey on a Gummi Ship trying to make a Gummi Highway and setting up a business empire. They had to part ways when the Heartless started to appear.
 Kingdom Hearts Birth by Sleep
Aqua arrives some time later in search of Terra. She encounters the passing Scrooge in the Central Square, and she asks him if he has seen a boy not from around there, impressing him with her polite manner. Scrooge mistakingly believes she is talking about Ventus, and he sends her towards the castle.
Ven chases the α Armor into the Central Square of Radiant Garden, he finds it accosting Scrooge McDuck, who is angrily swinging his cane at it. Ven drives it off with a Strike Raid, but when he heads after it toward the Castle Town, Scrooge stops him and offers to repay him. Despite Ven's protests, Scrooge beckons him closer, and he reveals not only that he can tell Ven is from another world, but that he is, as well. He explains that he had Merlin transport him from his home world in the search of new business opportunities. When Ven gets impatient, Scrooge gives him three passes to Disney Town, telling him to give two to "two grown-ups", and lets him go.
On return trips to Radiant Garden, Ven and Aqua can find Scrooge in the Castle Town. He asks Ven if he has been to Disney Town yet and enjoyed the Dream Festival attractions, and he mentions to Aqua that he is having trouble finding the right place to set up shop.
 Between Birth By Sleep and Kingdom Hearts II
Scrooge was busy with the King's help, setting up a transit system to further his business expantion through the worlds, using a Gummi Ship to due so. However, when the Heartless appeared in massive droves, this forced Scrooge to abandon his plans out of safety.
How he spent the decade between the games is unknown, but he didn't lose his wealth due to the drop of business between worlds.
 Kingdom Hearts II
Scrooge McDuck resides in Hollow Bastion during the events of Kingdom Hearts II, along with his three great nephews. He meets Sora, Donald and Goofy during their first visit to Hollow Bastion, Donald and Scrooge both surprised to see each other. He now owns part of the town and his house seems to be built above the synthesizing shop. Opposite of the shop is a large freezer where McDuck tries to recreate the original sea-salt ice cream flavor, but is having little success. Throughout the visits to Hollow Bastion, Scrooge becomes closer and closer to achieving his goal, eventually succeeding. Oddly, though, Scrooge's form of currency is not referred to as munny. If you want to play the Skateboard freestyle mini-game in Hollow Bastion, you have to talk to Scrooge.
In the end credits after Sora and Riku defeat Xemnas and return to Destiny Islands, Scrooge is seen with King Mickey, Donald, Goofy, Queen Minnie, Daisy, Pluto, the nephews, and Chip and Dale. This makes Scrooge one of the few neutral characters that can travel to different worlds. Also in the end credits, you can see Yuffie handing out Scrooge's ice cream to everybody.
Scrooge is the richest duck in the world, having gained his massive wealth from hard work and being "tougher than the toughies and smarter than the smarties and making it square," on the course of his many adventures in finding treasure and through mining and other business endeavors. He's a traveler by nature, journeying all over, and even between worlds, in search of treasure and ways of expanding his many businesses.
Having worked so hard to acquire his wealth, Scrooge has become rather stingy and greedy; he loves his money more than anything in the world and seldom spends any more of it than he has to. Still, he values honesty and fair play, and doesn't like to be in anyone's debt. Like his nephew, Donald, he has a temper, and can appear grouchy and selfish, but is initially a good-hearted person.
Scrooge, like the other members of his family, is an anthropomorphized duck with white plumage, four-fingered hands, light blue eyes, yellow-orange feet, and a bill of the same color. He has a thick, half-ring of ruffled feathers going around the back of his head, creating the image of an old, balding man. Scrooge's wealth allows him to dress in fine, stereotypically "rich" clothing. He wears a blue jacket with a folded-down, red collar, red cuffs, and a yellow button near the neck. He keeps the jacket shut with a red belt. A black top hat with a red band near the brim sits on his head, and he wears pince-nez glasses. He also wears red spats on his feet. Completing his image as an old, rich man, Scrooge walks around with a brown cane with a gold tip.
Debuting in the 1947 Carl Barks comic story, Christmas on Bear Mountain, Scrooge was originally planned as a one-shot character, but as Carl Barks quickly saw the potential for him, he began making further appearances and soon became a major character in Disney comics, going from supporting character in Donald Duck stories to eventually getting his own comic series in 1952.
Disney early on wanted to use Scrooge in cartoons, but apart from a brief cameo on the Mickey Mouse Club TV series, he remained a comics-only character until 1967, when the short Scrooge McDuck and Money made his "official" debut in animation. He was used sporadically in cartoons after that, his most notable role being in the 1983 featurette Mickey's Christmas Carol, where he played his Dickensian namesake, Ebenezer Scrooge.
In 1987, twenty years after his animated debut and forty years after his comics debut, Scrooge became the main star (alongside Huey, Dewey, and Louie) of the animated series DuckTales, which loosely adapted many of the original stories from the comics.