But also think about it. Where is Onion in Chapter 4? Not at the pond. BECAUSE HE'S IN THE DARK SANCTUARY! Only somewhat /j, because this is a theory that can't be ruled out (yet). Just to expand on this possibility, I'll mention some more things that support the theory.
So, Onion is an octopus or a squid, right? You know what they can do? Squeeze through tight gaps. This ability could potentially translate into shapeshifting in the Dark Worlds. Why does he try to steal Toriel, and successfully steal Undyne? He's lonely. What if, inside the Shelter, is a pool of water that's connected to the lake (or whatever that is)? "Why does he make the Dark Worlds though?" you ask? Same reason as why he wants to steal Toriel and Undyne; he's lonely. He probably doesn't understand that making too many fountains would cause the Roaring, and/or Titans. Why he fled after the Titan formed in the Third Sanctuary? He got scared of the GIANT HAND AND LASER BEAM that appeared from the fountain.
TLDR; there's a lot of evidence that could possibly point to Onion being the Knight, though it's VERY unlikely. Feel free to say what you think about this theory in the replies, or just to crack jokes to build on it.
Between this and the Knight being someone who’s canonically missing and unable to contact the rest of her family yet can somehow afford to roam the Light World freely to make fountains…
This is more plausible.
Also the dark fountain background and what we see in the Goner Maker is based on a texture of the ocean.
Dark Fountain connected to the ocean? Could be a narrative clue to the Knight being connected to the ocean! And who do we only find in water? Who hears the sound of the ocean beneath the waters?
