Selph (also known as Self) is a deleted character (presumably a Nightmaren) who was originally going appear in NiGHTS into Dreams as a secret boss. Their existence is one of the most speculated topics about the NiGHTS series by the community.
In an interview made by fans, Takashi Iizuka, who was surprised when he found out that fans knew about the character's existence, said that Selph was intended to be a boss in the game, but they got deleted because Iizuka believed the game's story was already perfect, implying that Selph would impact the game's story. Yuji Naka revealed in an interview that Selph would be a secret boss who would be based on the self archetype from Carl Jung's works and theories about dreams, and described them as "Myself I could never be exceeded".
Files and Data discovered
Even through Selph was deleted, some of their data was found in the files by a hacker. Their name was found in a list nicknaming all of the game's bosses, being the seventh boss listed, after "M6:MONSTER-FISH" (Gulpo) and before "#M8:WIZED-MAN" (Wizeman).  Selph also already had a boss battle theme called "Know Thyself", which can be found in the game files.
A dataminer also found an unused icon, similar to the boss icons that appear on the save select and on the score display at the end of a level. This icon features a design that matches no existing boss, and thus it is possible that it represents Selph. If this icon is indeed Selph's, it is currently the only graphic yet discovered that shows Selph's appearance.
"Selph", however, might not be the character's actual name, as Selph's name was found in a list from the files which was nicknaming all of the game's bosses, with Gillwing being nicknamed "#M1:FROGED-DRAGON", Puffy being "#M2:MRS.FATTY", Clawz being "#M3:FIRE-CAT'S", Jackle being "#M5:MANTLE-MAN", Gulpo being "M6:MONSTER-FISH" and Wizeman being "#M8:WIZED-MAN".  Selph is simply called "#M7:SELPH", indicating that the name "Selph" could actually be a nickname or a provisional name for the character. The list was most likely made at the very beginning of the game's development, back when none of the characters had an official name and these nicknames were simply how they were addressed.
On December 4th, 2021, a dataminer found an unused nameplate with the word "Self" in it. This nameplate is similar to the nameplates with the names of the bosses that appear in the game's credits. This of course means that Selph, despite being a secret boss, would appear in the game's credits and that the character's true name may actually be just "Self".
"More NiGHTS" icon
In the NiGHTS into Dreams official Japanese website, there is an icon with a never seen before character's head and the message "More NiGHTS" displayed under it.
The character looks similar to NiGHTS and Reala, but has different colors (golden and purplish blue). Their head and hat shape are also unique in that they curve to the sides rather than the back.
Four of these eight unused head items also resemble an unknown never-before-seen character, who appears to be wearing a hood and has a diamond-shaped gem on their forehead, one of the heads even has a red colored diamond-shaped gem which looks very similar to the red gem onto NiGHTS's chest. This unknown hooded character is also speculated to be what Selph would look like.
Despite the unique design of the character in the "More NiGHTS" icon, it is possible that the character is just NiGHTS, as the head appears in some of the game's ads. It is also possible that this golden head with unique hat shape curving to the sides was simply created for game advertisement.
The head of the unknown mysterious "hooded" character with a diamond-shaped gem on their forehead was currently not seen anywhere else though, neither in the game, nor in game's advertisement.
- For years, there was a rumor that Selph had a mirror staff, and parts of said staff were found in the game files. However, this rumor was confirmed to be fake.
- In the game's files, Twin Seeds is nicknamed "#P7:ELIOT'S-CITY" and there is a deleted level nicknamed "#P8:SPIRAL-CLARIS".  This suggests that Twin Seeds was originally going to be a level exclusive for Elliot, and that Claris was intended to have her own unique final level. Some fans believe that the deleted level would be where Selph would appear as a boss. However, Yuji Naka confirmed in an interview that Selph was actually going to be a secret boss in the game, indicating that Wizeman was most likely the boss of Spiral Claris.
- In an interview, the boy who sang the kids version of Dreams Dreams in NiGHTS into Dreams, Cameron Earl Strother, revealed that the developers told him that the game would have a secret boss when he went to Sega to record the song, and he was mildly disappointed when he found out the boss was cut when he played the game. It is unknown if Cameron was simply told about Selph's existence or if he actually learned about what Selph and their boss battle would be like.
- Selph may not be an actual nightmaren, as they would be based on the Self archetype from Carl Jung's theory about dreams, which has traits that would be strange for a nightmaren. The archetype is said to appear in dreams in the form of royal figures/divine beings and is expressed with numinous aura around them.
- There's an unused audio file that can be found in the game's data of a man saying something which sounds like "sooooora". This audio file is speculated to be a Selph audio file. 
- It should be noted that this audio file is located in many sound files linking to multiple parts of the game, which lessens the likelihood of it belonging to Selph, as if the audio file were Selph's, it would only link to Selph's file.
- According to Jung, the Self archetype appears in dreams as royal figures, divine beings, mandalas and holy items.
- The archetype of Wise Old Man could be considered to be one of symbols of the Self, meaning Wizeman was probably related to Selph.
- A hacker found almost all of the music tracks from NiGHTS into Dreams in the NiGHTS: Journey of Dreams files, including Know Thyself. However, considering all of the NiGHTS into Dreams boss themes were found as well, it is safe to assume Selph was not planned to appear in the game.
- Possession by the Self and mana personalities, that are lower expression of it (Great Mother and Wise old man) causes megalomania, aka delusion of grandeur which "Myself I could never be exceeded" probably meant.
- Self is the most famous deleted/unused Nightmaren.
- Unnamed Scrapped Nightmarens: List of other nightmarens who were scrapped or disconsidered during the game's development.