Chara = fallen human
Fallen human- fallen angel
Fallen angel = the devil/ demon
Chara = demon
Chara also = the Angel from the Delta Rune prophecy?
My interpretation of the Delta Rune symbol is that the three triangles on the Delta Rune represent the three candidates to be the Angel. Asriel, Chara and Frisk. Chara failed to free the monsters from the underground and so ‘fell down’ (died), failing to fulfil their role as the Angel in the prophecy and eventually became a demon, like the fallen angel.
The three candidates makes sense too.
In pacifist, Asriel breaks the barrier, though Frisk played a part in this and is the one who gets credited for it. This fulfils the original interpretation that the Angel that has seen the Surface (Frisk coming from the Surface and Asriel having seen it when attempting to carry out Chara’s plan) will free the monsters from the Underground by breaking the barrier.
Alternatively, in Genocide, the darker outlook of the prophecy is fulfilled, that darker outlook being that the Angel is an Angel of death that will ‘free’ the monsters from the mortal realm by slaughtering them. This is the entire plot line of Genocide, carried out by Frisk with help from the demonic spirit of Chara.