She's the identical twin to my main (well 2 mains now for I enjoy the Arcanist too), Blue-Thuxis, my Argonian werewolf (yes, my Arcanist shares this same Hircine blessing) Sorcerer, who both born under the Serpent birthsign. Unlike their siblings from the same clutch of eggs, they hatch out of the same egg. (Like the Argonian twins from Oblivion (Ocheeva & Teinaava) would say "egg-mates".)

Though base from the Tree-Minder of the Hist said it predicted to be only one supposed to came from that egg...

Cobalt was male like her brother, though she has spines (Fine Upstanding Spines) while Blue who has feathers (Maximum Frisee later Misanthropic Lycanthrope) on the head, others still confuse them! So she changed her gender to hope that reduce the confusion, it did, mostly. They have the same interests of like in clothes, armor etc.. They both are skilled at dual wielding while also with a bow & a destruction staff. They do has a strong telepathy connection, while Blue the one was called the vestige during the escape of Coldharbour she was there as many other soul shriven who escaped as well. But after the portal they got separated, she ended on the Isle of Balfiera! But eventually find her brother & siblings. While she not present during most of those game story past events, she experience it through their connection, seeing it through his eyes.

Though, headcanonly, she not too far. For they do travel together unlike their siblings they sometimes run into, Crimson-Thuxis (a brother, DragonKnight) & his wife Urmanda (trans female Orc, a Warden), Ros-Ei Nature-Thuxis (a sister. a Warden), & Bone-Thuxis Tharn (a brother, Necromancer. His last name is due when young he got taken by Dres. A Imperial ,knew he couldn't take on all of Dres or able to rescue all, he only able to buy Bone's freedom! Bone thought his mother & sibs were killed. So the Tharn adopted him.) Other they run into are their own mother Nirn-Scale Rajthux (Templar, old but she doesn't look it, still good in a fight) & Selene Gray-Thuxis. (Nightblade, due having "Thuxis" she's not a relative but a Shadowscale friend they met)

Rumor (or is it?):
Back to unrumors (or was the above is fact ), the tome that came to Nirn that looking to bind itself with a mortal soul, both Argonian twins souls were perfectly matches the tome's arcane resonance! The tome literally couldn't decide, it ended choosing Cobalt-Thuxis to bind itself with, unknowingly it got itself in a 2 in 1 problem. While Cobalt-Thuxis the one became a arcanist & got align with Apocrypha, for both twins are mind & spiritually connected her twin can experience what's happening! But due to her twin bond, it prevents Cobalt having a exact innate connection with Hermaeus & the affects like other arcanists! She got the powers of a arcanist, but the so "complex runes, tentacles and abyssal waters" doesn't rush around her head. Like both helped each other with sorcery spells, Blue-Thuxis helped his twin to practice her new craft! Through the Necrom chapter she's going to be the hero this time around!

& that's it, a wild ESO headcanon background mentioned 9 of 14 of my toons.