Introduced in Ultimate Custom Night, this character never given a proper name, but it only referred to as "The One Should Not Have Killed" by the other cast members. This character is a spirit and is the reason why the events of UCN happened.
This character is very vague, so details are sketchy. Here's what we know about this character both in and outside of UCN.
This character is assumed to be the one who was possessing Golden Freddy. After getting all the cutscenes in the game, you can see Golden Freddy twitching as he fades into the darkness. Nothing else beyond that.
Furthermore, some of the lines from the Medicore Melodies also help to imply that they were one of the possessed children who burned in a fire.
In some of the lines from the Medicore Melodies listed below, you can hear a second voice under neath them if you listen closely. That second voice belongs to "The One You Should Not Have Killed".
Happy Frog: "We’ve only just begun. I will never let you leave. I will never let you rest."
Nedd Bear: "This is how it feels, and you get to experience it over, and over, and over again. Forever. I will never let you leave."
Orville: "He tried to release you. He tried to release us. But I’m not gonna let that happen. I will hold you here. I will keep you here no matter how many times they burn us. "
Listening closely at the voice, it sounds like a young girl. This led to the common belief that this spirit is a female. However, there is some contradicting evidence in the game itself, as Withered Chica and Mangle use "he/him" pronouns for "The One You Should Not Have Killed", the same spirit.
Turns out that this spirit is indeed a male, as the voice listing posted by Scott (referring to this character as "Vengeful Spirit") before the game was finished confirmed that it was a red herring.
"This is for the voice of a young child, who speaks in a whisper from the shadows. This child is in control, and is toying with the player, who is helpless to change their situation or prevent their inevitable end. The gender should not be immediately clear, it should work as either a young boy or a young girl, and you are welcome to do readings leaning one way or the other."
-Scott Cawthon
Despite the gender being listed as "female", this was a complete listing as taken by Tabatha Skanes. Of note, this job listing was posted on the same day as the other voice lines for the Melodies.
Furthermore, Withered Chica's Job posting was submitted on May 28th, 2018. The listings for the spirit and the Melodies was posted on May 8th, 2018. This means that the lines for Withered Chica and Mangle saying that the spirit is a male was planned from the start and Scott simply rolled with the female voice chosen for this character.
Of note, the job listing has 300 words for this character. This implies that there were more voice lines that didn't make it into the final cut, assuming that it would be used for the aforementioned Golden Freddy cutscene.
The Fazbear Frights books shed more details on "The One You Should Not Have Killed". In the books, the Vengeful Spirit is a boy named Andrew and used to possess Golden Freddy until it left to torment William Afton, whom "The Man on Room 1280" confirmed that UCN was a nightmare.
Taking that information into account, this further adds to the "Vengeful Spirit is a Male and Possesses Golden Freddy" points.
Considering that The Fourth Closet, a dubiously canon book shed some light on the names of the missing children in FFPS and how the Survival Logbook is connected to the game lore, it's possible that the actual name of "The One You Should Not Have Killed" is Cassidy. In the Charlie Trilogy, Golden Freddy is possessed by Michael Brooks, making Cassidy possess either Freddy or Bonnie. In Fazbear Frights, Andrew's body was inside Golden Freddy before his soul left to torment William Afton, his killer.
The Updated Freddy Files hinted that the covered up grave in FFPS belongs to Cassidy, whom possessed said animatronic in the games. The Updated Freddy Files strangely advertised the Logbook in that part, further suggesting this.
While Cassidy is a female in The Fourth Closet who does not possess Golden Freddy, Cassidy in the games is a boy who possesses Golden Freddy.
Who Cassidy really is in the game lore is left up for interpretation.