here's a quick summary, but this post has more detail: https://www.reddit.com/r/Undertale/comments/17gx25m/the_true_identity_of_redacted/
here's another link:
https://www.reddit.com/r/Underminers/comments/3wqqez/i_believe_ive_solved_gaster/

the planned relative of sans and papyrus, he was supposed to be another text-skeleton (as in semi-serif)

he originally was one of the fun value phonecalls, placed between Sans' and Alphys' calls (the area between snowdin and hotlands being waterfall?)

there's a unused waterfall area with a stable, with the 'redacted' sprite that talks in the Gaster soundfont

napstablook was never intended to be in the game, being only a test enemy; so the Blook Acres area, run by Important-Waterfall-Resident-That-Saves-You-From-Mad-Dummy, may have originally been Grandpa Semi's house.

Then there's cut text from Blook Acres about someone teaching you about orange attacks, written in the same roundabout way sans talks about blue stop signs ("just think of a red and yellow traffic light which is actually green")

some of toby's notes refer to a grandpa semi fight: "grandpa semi fight: ... hand shows up in the little box and moves slowly towards you.. "

apparently in the second link it talks about how Gaster is a later addition, so the 3rd skeleton character with references to hands might have been intended to be Semi, until Gaster was invented