I think that the knight is still someone we know (pretty sure it's implied they're a lightner).
Also multiple knights theory is debunked
And I believe there is now only ONE (maybe two) possible options based off the following.
Susie mentions that since the knight has a connection to the mayor, the police, and whatever the heck the third thing is.
THAT MEANS THERE ARE ONLY TWO POSSIBLE PEOPLE
The Mayor herself (though the connection to the police is more vague, maybe the whole bunker thing idk)
OR
Well
You know what I'm gonna say.
Disgraced ex-police chief, family friend of the mayor and best friends with the mayor's husband and all that jazz.
And I feel like it's being hinted at so clearly. Kris tells the knight to come over (oh yeah kris knight kahoots what the hells going on there?) and then we see asgore, shortly followed by mayor holiday. The scene is played so that at first you think it's asgore, and OMG asgore's the knight, but the mayor holiday arrives home from work early, implying that SHE'S the knight.
BUT
BUT
BUT
There is NO possible way that the mayor could have opened the church dark fountain.
She was with noelle at the time, or at least inside the house.
BUT YA KNOW WHO WASN'T IN THE HOUSE?
YOU KNOW WHO WE KNOW WAS BY THE CHURCH, LURKING IN THE BUSHES?!
MR. FLOWER KING HIMSELF.
and if THAT isn't enough...
there's one DAMNING piece of evidence left.
WHO is the Knight obsessed with?
Kidnapping?
Capturing?
WHO DO THEY SEEK?
Tori.
I rest my case