Ninja Gaiden Ragebound has 3 Endings: Ending A, Ending B, Secret Ending. The Secret Ending is only available in Hard Mode.
Endings A & B depend on whether you pick Kenji or Kumori to escape hell in Level 4-6: Into the Shadows.
The recommended approach is to go for Ending A on the first playthrough, then replay the final level and combine Ending B + Secret Ending, since Ending B is needed to unlock the Secret Final Level. You are allowed to enable Assist Mode in the game settings and set incoming damage to 0% to become invincible, this doesn’t disable trophies/achievements and makes it very easy.
Ending A
To earn Ending A, you need to complete the game up to Level 4-6: Into the Shadows. Once you reach the boss on this level, the game will ask, “Who will escape Hell and seal the door?” – make sure you choose Kenji!
Some players are also getting the Ending A trophy when picking Kumori on their first playthrough, which might be a difference between PS4/PS5 version but as indicated by the trophy image Ending A is supposed to be from Kenji. If you get Ending A by picking Kumori you can simply replay the final mission and pick Kenji to unlock the other ending trophy.
Ending B
To earn Ending B, you need to complete the game up to Level 4-6: Into the Shadows. Once you reach the boss on this level, the game will ask, “Who will escape Hell and seal the door?” – make sure you choose Kumori!
Secret Final Level Ending
To unlock the secret final level, which grants you the game’s third ending, you must first complete the game in Hard Mode, including all the main missions up to level 4-6: Into the Shadows. Once you reach the boss on this level, the game will ask, “Who will escape hell and seal the door?”
Make sure you choose Kumori so that Kenji remains in hell!
After the credits roll with Kumori, the game doesn’t end there. You will now control Kenji in Hell, where you must face your unfinished business. This unlocks a boss rush mode in which you fight all the previous bosses one after the other.
This is where the game reaches its peak difficulty. If you don’t activate Assist Mode and set received damage to 0%, this will be the hardest challenge in the game. You cannot heal between boss fights and most bosses can kill you quickly. You’ll need to play almost perfectly to succeed.
Once you have defeated all the bosses in this final gauntlet, you will unlock the ‘Endless Struggle’ trophy and see the third and final ending of the game.
Hard Mode features reworked levels with new enemies, more traps and increased damage. If you choose not to use Assist Mode here, you’re in for a real challenge, as this mode pushes players to their limits.
To combine a playthrough with 100% collectibles, refer to Ninja Gaiden Ragebound Collectible Guide, and for trophy cleanup check out the Ninja Gaiden Ragebound Trophy Guide & Roadmap.

Schlyter says
I got ending B when selecting Kenji during the first playthrough
LeoOne BR says
Yeah same for me. Got final A playing on normal mode and chosing Kumori. Played again the last level on PS5 andgot same results.
LeoOne BR says
*Ending B in my last comment