I bring an ancient idea for an ancient post.
Yes, the four Circles do have "sources of light" as a common theme.
Sun and Star are technically redundant because they're both stars, but they serve two different uses throughout history.
Moon doesn't produce its own light but it does give us just enough light to see at night, and guides us in the same way that stars can.
Lightning is not a celestial body but it is able to create fire (so fire-user MCs are a branch of Lightning Circle, and still are if you ask the ATLA universe) and is considered a divine weapon in mythology.
So if anything, they're chosen as much for their distinct symbolism and real-life uses as they are for their uniting concept of light.
The implication that the 5th Circle's symbol is a butterfly (or Highhorse's fire or spiral) is a very sound one given the elements the game has presented us with so far. So much so that it likely is what SSO intended for us to assume.
But may I present Bioluminescence for consideration.
Like the butterflies, it is right in front of your face in-game. But unlike the butterflies, it is a source of light in real life.
Does it have a "god(dess) of" level of mythological presence? Not really, but that doesn't stop it from carrying a wide range of symbolism. Plus its status as a lesser light gives it a "dark horse" quality.
It's not as bright or grand in scale as the other four, but it can light the earth when the other four can't, and can be seen from space if allowed to grow big enough.
Bioluminescence depends on a life force to exist, so in a way it is a sentient, living light (this is your reminder that humans too are bioluminescent, just not on a level we can see with the naked eye).
It does seem to have at least a tiny amount of metabolic expense so there's that aspect for fans of magic having a cost. That cost may or may not increase proportionally with the size of the host since entities that have it irl are much smaller than a human.
Bioluminescence is unique to Earth (so far) so that gives it a more direct connection to Mother Nature, and by extension the soul of Jorvik itself. Though it is found more in the deep ocean than the open air.
Only Lightning can compete with it to reach the darkest depths of the sea, let alone constantly exist there.
In nature, it is used for defense, as a lure or attraction, and as a form of communication in a species.
It can also be toxic or destructive (e.g. algae blooms, and can create and deplete oxygen).
It objectively looks the most haunting and magical of all five light sources, especially through fireflies and plankton.
Speaking of fireflies and symbolism-- they're associated with divine guidance/intervention, hope, love, resilience, nature spirits (fae, will-o'-the-wisp, etc.), and the souls of the dead, among other things.
Fireflies are also called lightning bugs or moonbugs, and are compared to stars as a "living constellation", so bioluminescence can connect to at least three of the four circles. Which may mean Anne would have the least in common, or the most appeal in a "novelty" way, to a Firefly MC.
There's probably more that bioluminescence has going for it but that's what I've got off the top of my head.
As far as a Circle symbol goes, a firefly might be your best bet. Or at least it's a solid alternative if you have an MC you headcanon as standing on their own, unconnected to Catherine.
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