Kingdom Hearts bosses come in all shapes and sizes and have varying degrees of impact on the series whole. Whether it be because of their musical scores, difficulty, or emotional weight, most if not all bosses tend to leave a lasting impression on the player. By incorporating at least one of the mentioned elements, a boss gains an intensity that drags players deeper into their respective game's experience, fully immersing them in it.
However, some of these head honchos do it better than others, as they manage to incorporate all three elements into their encounters, and as a result, they generate much more intensity than their peers. Five boss battles that happen to fulfill these conditions come to mind; although, more certainly exist throughout the Kingdom Hearts series. Between the five, some use their elements better than others, but still, in no particular order, they include:
Ansem-Riku (KH1)
Ansem-Riku results from the tensions built up between Sora and Riku since the start of Kingdom Hearts. After presenting the relationship between the two as friendly and competitive, the game slowly breaks them apart after Riku feels Sora replaced him and Kairi with Donald and Goofy. The misunderstanding leads to recurring ideological clashes between the characters, creating an opportunity for Heartless Ansem to manipulate Riku into giving over his body and into dueling Sora over the fate of Kairi's heart.
Not only does this fight rift a lifelong friendship between three characters, but and epitomizes the concept of a deathmatch. To this day, it utilizes the smallest arena in Kingdom Hearts, leaving no room to run, hide, or even breathe. There's just Sora, Riku, and the idea that only one of them will be walking away. Its music, Froze Del Male, perfectly encapsulates these concepts by giving the fight both an intense and foreboding ambiance, and Ansem-Riku certainly makes it one of the most difficult fights of KH1.
Terra-Xehanort, 1st Battle (BBS)
In Kingdom Hearts Birth by Sleep, Terra found himself in a place with two father figures. In Eraqus, he had one who didn't understand him but still loved him and wanted what was best for his development. In Xehanort, he had one that understood him completely but wanted to use that knowledge to exploit him — and it's this father who succeeds. Xehanort orchestrated a situation that led to Terra assisting him in killing Eraqus, breaking Terra's heart, and allowing Xehanort to possess his body. What follows is a final clash between the two.
What makes this fight so intense is Terra himself. His understandable anger at Xehanort's actions is so palpable that it physically animates his discarded Keyblade armor. His rage creates the Lingering Will, whose purest instinct is to destroy Xehanort. It's like a ghost from beyond the grave haunting the individual that wronged them, and the fight itself is given adrenaline-pumping energy with the aptly named musical piece "Rage Awakened."
Terra-Xehanort Finale (BBS)
Terra-Xehanort's second battle, but this time Aqua steps to the plate. After Lingering Will fails to stop Terra-Xehanort, Terra — losing the battle for control over his body to Xehanort — calls out to Aqua and asks her to destroy them both. Aqua traces this call back to Radiant Garden, where she learns of Terra's possession and challenges Xehanort with the intent of helping Terra free himself. Should she fail, then she'll have lost everything, both her best friends and her teacher — and all after having just achieved her dream of becoming a Keyblade Master.
It's the eminent tragedy and the unease that it generates that makes this fight so intense. Since none of Birth by Sleep's main cast appears in Sora's storyline, it's clear from the beginning of the game that all of them will suffer some kind of tragedy before everything is said and done, and by this point, Aqua's the only domino left to fall. Eraqus is dead; Ventus unwillingly put himself in a coma, and Terra will go on to become Heartless Ansem and Xemnas. So what happens to Aqua? The fight's background track, "Dismiss," captures the fight's ominous atmosphere perfectly.
Final Form Xion (358/2 Days)
When Xion realizes the truth behind her identity, she finds herself left with one very difficult decision: return to Sora. Organization 13 created her as a faceless puppet with the ability to trap Sora's most important memories — his memories of Kairi — inside herself. Without them, Sora can't wake up from the sleep Namine put him in, and as they are the foundation of Sora's being, the memories will inevitably cause Xion to destroy Roxas so that they can reunite with the memories he has trapped inside of him.
Xion can't let that happen as Roxas is her best friend. But the only way to stop the process is for her to return the memories to Sora, and the only way she can do that is if Roxas destroys her first. What results is probably the saddest fight in Kingdom Hearts, as Xion forces Roxas to kill her in self-defense. It's a hard boss fight to follow through with, and its musical track "Vector to the Heavens" reflects both the intensity and sorrow behind it.
Final Xemnas (KH2)
This is perhaps most famous fight in all of Kingdom Hearts. After reuniting following their rift in KH1, Sora and Riku, along with Kairi and the rest of their friends, attempt to finish off Organization 13 for good by defeating Xemnas. Circumstances separate the group and see Sora and Riku fight Xemnas alone across a wide variety of forms until the power he absorbed from his artificial Kingdom Hearts settles on one final transformation. Trapped in the Realm of Nothingness, the duo fight Xemnas while he's in this form and do so with the understanding that they will likely never see Kairi or any of their other friends again.
There are several reasons why this fight holds the title as the most famous in the series, such as its unrivaled action setpieces and the haunting way its track, Darkness of the Unknown, captures the ethereal nature of both Xemnas and the fight itself. But the highlight of the fight is Sora and Riku's team-up. The two working together is a complete flip of their situation in KH1, and when combined with the other mentioned factors, it generates a level of hype and excitement that eclipses every other Kingdom Hearts boss to date.
Kingdom Hearts is on PS4, Xbox One, PC, IOS, and Android.