The names of two main characters of the game - Frisk and Chara, while technically correct, sound very... unnatural. So I've thought - what if they're actually a result of letter substitution (with maybe some small modification for readability) for their actual, hidden names in the lie of the "cave is actually hell" theme?
Then I've thought - there should be some in-game hint for that, a codeword. After I've thought about that a little, it came to me - that codeword was the name Frisk. Why else it would be so difficult to actually see, requiring the True Pacifist ending to achieve?
But what does Frisk maps to? That one was obvious - Human! In-game characters constantly call you "human", and indeed it could be achieved as a result of letter substitution.
The letter substitution table you get from "Frisk - Human" looks like this:
f <-> h
r <-> u
i <-> m
s <-> a
k <-> n
Then I applied it to Chara, and see what you get:
c <-> ? (unknown)
h <-> f
a <-> s
r <-> u
a <-> s
(Chara <-> ?fsus)
This doesn't look good at first sight, but let's continue. What does the letter C maps to? Let's look at the relative distances between positions of substitute-substituted letters:
a (1) <-> s - (19) - 18/8 (depends which way you're counting)
r (18) <-> u (21) - 3/23
i (9) <-> m (13) - 4/22
k (11) <-> n (14) - 3/23
h (
<-> f (6) - 2/24
Let's check what happens if we apply the (3/23), as the most common shift in the list, to letter "c":
c (3) <-> f (6)
Nope. But if we try (4/22), as being close enough to (3/23) and also being in the "Frisk - Human" substitution list:
c (3) <-> g (7)
And since "g" and "j" are very close sounding letters, you get the "Chara <-> "Gfsus" ~ "Jesus", if we shift first two letters back by one (1). You know, so that the names sound human-ish.
Something tells me this is very much NOT a coincidence.