While most people think of the Kingdom Hearts series as a fun mashup, hardcore players know just how unforgiving the games can be. Kingdom Hearts gives most players the option to play through its stories on standard mode or lower.

The most seasoned players will choose proud mode and are sure to tackle every secret boss the game has. This leads them to find some of the most frustrating bosses in gaming. From the notorious fight at Olympus Coliseum with Sephiroth to the Mysterious figure, there are a ton of in-game fights that have made even the most passionate players lose their cool.

10 Vanitas Remnant

Vanitas Remnant is a secret boss in Kingdom Hearts: Birth by Sleep. This is an Unversed that can be found in the Keyblade Graveyard’s Badlands by interacting with a mysterious sphere. This secret boss can only be accessed after the player has beaten the game and is a much harder version of the Vanitas fight.

The creature is said to be the feeling that Vanitas himself left behind after being defeated. This boss is quick and hits hard, while the fight disables D-Links.

9 Data Marluxia

Data Marluxia appears only in Kingdom Hearts II Final Mix. As most players already know, data fights are a harder version of in-game bosses and can be quite the hassle. The Data iteration has all the skills of the original Marluxia plus the ability to surround players with thorns to deal damage.

Data Marluxia also has more health than the normal version and cuts Sora’s Restore Count in half when it is used during the fight.

8 Dark Inferno

Dark Inferno is a secret boss located at Battlegate 14 and has a less powerful form found in Kingdom Hearts III Re Mind. Like with most secret bosses, once you have knocked down Dark Inferno’s health, it will become more aggressive with its attack in an attempt to kill Sora.

Dark Inferno has attacks that can’t be blocked and can stun the player, leading to plenty of frustrating situations.

7 Data Xemnas

When the DLC of KH3 released, many players found themselves stuck on certain data fights. Xemnas is one of them as the character comes back with a two-part battle, where the player must juggle as playing as both Riku and Sora without dying in either of their parts.

In addition, Xemnas also changes his attack patterns from his previous encounter to further trip up players. Data Xemnas is considerably harder than any boss fought before him in Kingdom Hearts 3.

6 Data Master Xehanort

To beat Master Xehanort’s data form, you will have to perfectly follow a pre-planned attack plan for each of his phases. Most players who go into the fight blind end up completely overwhelmed by his multitude of attacks and will fail even after using a Kupo Coin.

Between dodging ice bombs, lasers, and spheres, it can be pretty hard to keep a full health bar against Data Master Xehanort.

5 Sephiroth (Kingdom Hearts)

Sephiroth is one of the most notorious boss battles in the gaming world. Sephiroth appears in the very first game in the series as a secret boss who is nothing short of brutal. Sephiroth could take down maxed out characters in a matter of seconds without Sora ever landing a hit.

To make matters worse, the battle was even harder to get through when the game debuted due to a lack of videos and proper guides.

4 Unknown

Unknown appeared in the first game and is considered by many gamers to be even harder than Sephiroth. This is due to the fact that he was an incredibly fast opponent that can even summon a force field to push back Sora while dealing damage at the same time.

To top it off, Unknown also has a one-hit kill ability called shock that forces players to make a lighting quick choice.

3 Mysterious Figure

Mysteries Figure appears in the nonfinal remix version of Birth By Sleep, where he can be found in the Land of Departure. He can not be affected by status effects and his attacks can be a bit random, making Mysteries Figure an enemy that is extremely hard to predict. He even had to be toned down for the Final Remix version of the game.

You will need to get familiar with perfectly timed cures and being able to dodge attacks on the fly to stand a chance against Mysteries Figure.

2 Lingering Will

Lingering Will is in the Final Mix version of Kingdom Hearts II. He can be found inside of a portal located at the Disney Castle. Lingering Will uses many of the same combos as Sora and can be hard to hit as he doesn’t leave many openings for the player to attack.

On top of that, Lingering Will can use lasers, magic, and has the ability to kill Sora in one shot with his attacks.

1 Yozora

Yozora is the boss fight that no one was expecting at the end of Re Mind. Yozora is the most merciless boss fight in the series so far as he can cut down maxed out characters in a matter of seconds. Yozora has a huge array of deadly close quarters and long-range attacks to take on Sora’s many different abilities.

To make matters worse, Yozora even has the ability to steal the player’s Kupo Coin and use it to heal.

NEXT: Kingdom Hearts: Top 5 Strongest Abilities In The Series (& 5 Worst), Ranked