As a starting point, I decided to focus on celestial bodies as a "theme" for my characters. I chose 4 and then made initial notes on each as if I were writing for a character.
For placeholder names, I will refer to them as Luna, Gwen, Eris and Pluto.
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Luna is based on the moon, and I have a few other possible names for her; Mona, Selena, Cynthia, Ina and Sin.
Her mark would allow her to control water, and when the mark becomes unstable it would make her body experience a lunar eclipse (her body would go pitch black). She'll be the first character to have her mark go unstable (since some people believe the moon is linked with madness). But she's also the first character to learn to control her mark.
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We then have Gwen, they're based on Venus. Possible names for them are; Terra, Maxwell, Coro and Novae.
Their mark allows them to control fire (since Venus is significantly hotter than Earth). And as a reference to the fracture like systems on Venus' surface, when they go unstable their body will look as if encased in spider webs. This character will also be Agender.
Luna and Gwen will be fraternal twins.
--- Admittedly I don't have as much for Eris and Pluto yet. ---
Eris is based on the dwarf planet of the same name. Possible names for her are; Xena, Lila and Persephone
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She will be able to control light with her mark, and when she goes unstable her eyes turn black
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Pluto is based on the dwarf planet of the same name. Possible names for him are; Urbain, Zeus and Atlas.
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Next I'll make some mood maps for each character.