Selph was a deleted Nightmaren boss from the original game, NiGHTS into Dreams. It was intended to be in the game but was deleted from the roster at the last second because Yuji Naka believed the game was "perfect enough".
Selph was supposed to be Claris' final boss, which was supposed to take place after an entire level that was deleted, known in the files as 'CLARIS_SPIRAL'. Claris was never originally intended to appear in Twin Seeds, but was shoehorned in after Selph was cut, since there wasn't really anything the team had time to do for her final level and boss battle.
Even through Selph was deleted, a hacker discovered some of the Selph's data in the roster, right before Wizeman's. This jewel of information included a music track of what would have been Selph's battle theme, entitled "Know Thyself," which bears an interesting resemblance to "Dreams, Dreams," leading many fans to believe that Selph has a strong connection to NiGHTS. Furthermore, on the disc for Christmas NiGHTS into Dreams, the menu image for NiGHTS notably has a slightly distinct design from the "real" NiGHTS, leading some to believe Selph may strongly resemble NiGHTS. However, no data exists relating to any physical designs for the deleted boss, leaving fans to speculate and invent possible designs for themselves.
Another nugget of information discovered were parts for a sort of "mirror staff" and additional information regarding the data, found by another hacker.
Selph's AI was supposedly used to make it possible for Claris to appear in Elliot's Wizeman battle, which leads fans to believe that Selph would have had a battle similar to Reala's.