So, here's my idea. Depending on how things go, I may do a CS for her sister as well:

[color=7bcdc8][b]• Name:[/b][/color] Linea Milsteen
[color=7bcdc8][b]• Age:[/b][/color] 15 years
[color=7bcdc8][b]• Gender:[/b][/color] Female
[color=7bcdc8][b]• Race:[/b][/color] Human
[hider=• Appearance:]
[center][img]https://image.ibb.co/kSAGf8/ciel_sacred_original_and_shirokami_project_drawn_by_mishima_kurone_1b0e58d3f4d6e873fd2452d4edd083f9_edit.png[/img]
[color=7bcdc8][b]Height:[/b][/color] 149 cm | [color=7bcdc8][b]Weight:[/b][/color]  42 kg[/center]
[/hider][color=7bcdc8][b]• Personality:[/b][/color] Linea is like a cat, always trying to save her energy, but able to burst into action all of a sudden when needed, especially when said action involves mental challenges, which she's very fond of. In addition to that, Linea always speaks in a low, almost deadpan—or perhaps drowsy— sounding voice, which makes a lot of her comments and remarks come out as snarky, despite that not being her intention... some times.
When it comes to personal interests and hobbies, Linea has her sights set for a few things: books, adventuring, magic, and first of all, her sister. Linea is also very competent in other areas, like cooking, but it's not like she would rather spend time in the kitchen than in a library. Unless, of course, she's cooking for her sister.
Just don't ask her to do anything involving physical effort. Mages have the physical prowess of a wet noodle, after all.
[color=7bcdc8][b]• Brief Backstory:[/b][/color] Both Linea and her two-years-older sister Sienna grew up at a church-sponsored countryside orphanage, after losing their parents at an early age. Despite what may be common thinking in relation to the lives of orphans, both of them never had any specific trauma or faced anymore hardships than other children of comparable age and social standing.
In fact, the single Mayonite nuns that ran the old orphanage always treated the children well and, despite the occasional bullies, all the children where taught to at the very least be polite towards one another, even if they didn't want to make friends.
As the two of them slowly came of age, and saw their older friends and acquaintances leave the place they began to call home, the siblings grew aware that they too would soon have to leave it behind, at least partially. Both joined a magic college and began taking lessons in the arcane arts. However, while it became clear that both had a natural knack for the Arts, Sienna's brash personality made her seek a bit of "side training" with an explorers guild.
Both sisters, one being an up-and-coming mage apprentice and the other what may as well be called a spellthief, took a life of adventuring together, being more experienced in dungeoneering and similar tasks than most people with the double of their age.
This path ultimately culminated with Linea receiving a recommendation letter from her tutor, pushing her to join the prestigious Iron Rose Knights as a trainee magic adviser. Whether or not Sienna manages to get a similar recommendation coming from her guild, Linea is sure that it's just a matter of time before the two of them are working together again.
[color=7bcdc8][b]• Equipment:[/b][/color] Linea's main noteworthy possessions consist of her grimoire, a crystal-tipped staff and spellcrafting tools and catalysts. She also always carries around a couple of journals and textbooks to assist in note-keeping as well as translation and decryption works, in case she comes across any mysterious source of information.
[color=7bcdc8][b]• Skills:[/b][/color] Despite still being an apprentice mage, Linea is quite capable in the spellcrafting arts, possessing knowledge of a good number of general purpose spells, that range from offensive to supportive in nature. Aside from her magic knowledge, Linea is also very good at gathering information, through means like research, translation and decryption of ancient texts.
Despite physical work not being her forte, Linea quite capable of taking care of herself in explorations or similar situations. However, she'll tire out pretty fast if she needs to do any form of prolonged effort, like rope climbing and running.
[url=https://www.youtube.com/watch?v=pJdPO74R_Uo]Nothing is stronger than the links between true family...[/url]
[center][img]https://image.ibb.co/kOdV7o/anceril_sacred_and_ciel_sacred_original_drawn_by_mishima_kurone_b9082df9211c87d3f82ae4424b4292f0_crop.png[/img][/center]