Doom Marine/Doom Slayer

The Doom Marine/Doom Slayer is an otherwise unnamed individual that serves as a driving force behind the events of Kingdom Hearts: DOOM as well as the main playable character in DOOM (2016). Unlike previous representations which were meant to be extensions of the player, the Doom Marine is established as a presumably ancient and mystical warrior that fights the demons from Hell either due to banishment or by choice.

Origins
All that's given about the Doom Marine was that at an unknown time him and a group called the Night Sentinels were sent to Hell due to a betrayal by one of their own (presumably by the Marine himself) which resulted in their main source of power only known as the Wraith being used against them. Eventually the Marine himself remained and fought until he was ambushed and imprisoned with the Night Sentinels' mark being used as a warning to keep him from being released. At an unknown time the Marine and the tomb he was imprisoned in was later recovered by Samuel Hayden during a UAC expedition into Hell.

Kingdom Hearts: DOOM
coming soon

DOOM
Sometime following the demonic invasion of Mars, the Marine was awakened from the tomb and started to investigate the cause of the invasion only to discover from Hayden and the UAC's Mar facility's AI VEGA that Lazurus Project head director, Olivia Pierce was responsible for the events prior to the Marine's awakening. He would later ultimately stop the invasion by closing the demons' source of power known as the Well along with defeating the Cyberdemon, a trio of demons known as the Hell guard, and Olivia herself after the latter being painfully transformed in the Spider Mastermind before being betrayed by Hayden and transported to an unknown location.

Personally
Despite the Marine's face never being fully shown in-game as well as not speaking (possibly due to while understanding the English language as seen from easily using terminals and following objectives given to him, may not being able to speak it), the Marine is more emotional compared to previous incarnations of the character. Examples of this can be seen where the Marine  angrily  destroys various panels when Hayden attempts to ask for assistance from the Marine as well as use him as well as when the Marine creates a back up of VEGA before detonating the AI's core despite not having been asked/told to do so. The Marine is also shown to be highly capable of using the hi-tech machinery and weaponary found in the UAC Mars faciity despite having no previous experiance/training with them.