Had to get a ref for Marina's eyepatch in a later part of my worldbuilding for someone. Than it turned into a lore dump. Here that is, heavily abridged, now!

Marina and Arcanica get sent to The Abyss (Hell) by an angel who is structuring covens around Etrium for conformation to the church (there's way more to it and its not just "The Church" but I'd be here all day). Arcanica, having turned into a Cambion (half-demon) due to more things I won't get into is on trial for her essence (soul) to be condemned to The Abyss under control under a Baron(ess) of The Abyss. Marina however, through more circumstances that requires to much context, proves she legally owns Arcanica's essence (soul) by higher rights in the cosmology of the world and thus it cannot be taken making the argument for her null and void

Arcanica, now free, is still stuck in The Abyss with Marina and is still a Cambion; however, as Marina won a battle of legal wit against a demon she is gifted (or cursed depending on how you look at it) with legalese demon eyes, which allows her to decern and solve any legal problem or loophole at a glance. This, however, ALSO makes her a Cambion. However,the demon who gifted her it, Baroness Xythillina, put to much 'oomf' into the gift and now the eye can not only do the legal mumbo jumbo, but it allows Marina to simultaneously see 10 seconds into the future through it and the present moment through her normal eye (kinda like Shulk from Xenoblade) The information of being in two places at once, one of which being a temporal offset dimension is overwhelming to her. Luckily however, covering the eye disables it. At the same time though, the eyepatch covering the eyes obfuscates the fact she's a cambion from regular individuals without the need for any type of illusion magic.

And THAT is why she has an eyepatch now :) (For this variant atleast, this is not the 'default' design and it's not changing).