Kingdom Hearts was a phenomenal gaming franchise, not only with enough titles to keep gamers busy for weeks, but they featured a groundbreaking combat system that still remains unrivaled for similar RPGs. Still, with all this going for the franchise, its main selling point was its inclusion of popular Disney characters. These characters were seen in a completely different light than in their respective movies and thrust into a whole new world that borrowed some of the best elements of the Final Fantasy franchise.
While Sora couldn't have gotten very far without Donald and Goofy by his side, Mickey Mouse played a huge role in saving the Kingdom Hearts universe (and some of its main characters) from succumbing to the countless forces of evil that constantly sought to destroy them.
After realizing that other worlds were suddenly vanishing, Mickey created the first-ever Gummi Ship so that he could travel to different places to get to the bottom of things.
Without his invention, Sora, Donald, and Goofy would've never been able to travel to different worlds later on, and the darkness would've eventually overtaken every single world.
In the first Kingdom Hearts game, Mickey remained largely an enigma despite being so universal in the public consciousness.
He was shown at the very end of the game, appearing on the other side of Kingdom Hearts to help Sora close it for good, and making a huge sacrifice by sealing him and Riku in the Realm of Darkness.
In Kingdom Hearts: Chain of Memories, Ansem's biggest goal was to take over Riku's body, since this would enable him to wield a Keyblade.
He made many attempts, and definitely would've succeeded if it weren't for Mickey commanding a powerful light that was strong enough to thwart every one of Ansem's attempts.
After Sora, Donald, and Goofy awakened in Twilight Town, the three struggled to recall how they'd ended up here, having forgotten their adventures in Chain of Memories. It wasn't long before they were swarmed by a few Nobodies that soon became more than they could handle.
Mickey swooped in out of nowhere and defeated the Nobodies before handing Sora a pouch of money that he used to board the train to Yen Sid's, which Mickey told him to board before running off. If it weren't for his intervention, Sora would've been defeated and Kingdom Hearts 3 would've never come.
Every Kingdom Hearts 2 fan remembers the first time they were fighting a difficult boss that they just couldn't beat: say Mulan's Shan Yu, or even the Organization XIII's Xaldin. Upon losing, they got the shock of their lives when they were prompted to select an option that read "I won't give up!" This allowed players to take control of Mickey, who could not only store energy to revive Sora, but could also do some damage to the opponent with Mickey's own overpowered set of combos.
The story never explained why he showed up in specific battles as opposed to others, but nevertheless, no fan argued with the chance to be able to play as him, since this option often enabled some players to emerge victorious from otherwise impossible battles.
Towards the end of Kingdom Hearts 2, after Sora managed to make it into the Organization XIII's hideout in the World That Never Was, Mickey went ahead to find Ansem the Wise, who'd already traveled to the hideout to stop the Organization on his own.
Since he isn't much of a fighter, Ansem had been swarmed by a group of Nobodies, but luckily for him, Mickey got there just in time to save him.
At the end of Kingdom Hearts Birth by Sleep, Aqua found herself trapped in the Realm of Darkness after saving Terra from such a fate.
Despite being a skilled Keyblade Master, the Realm of Darkness proved to be too much even for Aqua, and if it wasn't for Mickey showing up when he did, she would've been consumed by the countless Heartless that attacked her since she stepped foot in such a vile place.
In Kingdom Hearts 3, Riku and Mickey ventured into the Realm of Darkness to rescue Aqua. They didn't get far at all once the Demon Tower, a powerful accumulation of Shadow Heartless, appeared and broke Riku's Keyblade, threatening to consume him.
Though fans didn't see it, Mickey somehow managed to drive the Demon Tower back on his own, saving Riku from what would've been a certain death.
In Kingdom Hearts Birth by Sleep, Vanitas constantly sought Ventus out so that their union would lead to the creation of the X-blade. In an earlier encounter that they had in the Keyblade Graveyard, Vanitas had Ventus outmatched, and if it weren't for Mickey coming in and saving Ventus' life at the last second, he would've been toast.
Fortunately, Mickey's assistance was enough for the two of them to overpower Vanitas together, although his help arrived a little too late the next time Ventus and Vanitas clashed in the Keyblade Graveyard.
In Birth by Sleep, Ventus was unsuccessful in fending Vanitas off on his own, and was possessed by Xehanort's evil apprentice. He served as the final boss in Aqua's part of the game, wielding the powerful X-blade, which was too strong for any one Keyblade Master to fend off on their own. In working with Mickey, Aqua was able to break the X-blade, leading to a sudden explosion that nearly destroyed them all.
Somehow, Mickey was able to save Aqua and Ventus from a place Yen Sid called the "Lanes Between", bringing them to his tower so that they could receive proper care, or at least in Ventus' case, as much care as he could be given since his heart was lost as a result of his internal struggle against Vanitas.
