Overview
Rin comes across as a cold and logical young lady, but her aloofness hides a caring heart. As a magus, or practitioner of magical science, Rin can use Magecraft to various effects. In combat she can buff herself and others, fire hex bolts, manipulate base elements, and use jewels as exploding energy batteries. Her power is only average, but Rin's talent lies in finesse. After her father was killed in a battle royale among magi for a wish-granting relic, Rin and her sister trained to take up the mantle in his stead. Afterward they joined the Mage's Association in London to study. Rin has since been tasked as an official Association liaison to the World Tree, and she also acts as a token Association presence within Chaldea, an organisation monitoring humanity's survival.
Perks
Personal Resources: Rin has passive income, a nice house, and a motorcycle.
The Tohsaka family owns commercial properties, managing leylines to ensure supernaturally good business and therefore high land values and revenue. These, plus Tohsaka magical patents used by the Mage's Association, ensure Rin lives very comfortably. Her house is a large western manor atop a hill, warded to the teeth with generations of Magecraft. Its basement doubles as both a fully-stocked workshop, a comprehensive library, and an ironclad panic room. Lastly, Rin has an electric motorcycle to get around the World Tree. Aside from great mileage to a charge, it's unremarkable.
Magecraft (Enhancement): Rin can temporarily buff herself, allies, or objects with her Magecraft.
By infusing something with her mana, Rin can improve its natural properties. A blade will cut better. Shields will be stronger. Legs can be made to run faster. Fists hit harder. With Reinforcement, everyday objects can become good, combat-ready tools. By turning the technique on her own body, Rin can temporarily perform at superhuman levels of either agility, precision, or strength.
Martial Arts: Rin knows how to attack or defend with her bare hands.
Trained from youth in a style of martial arts favouring finesse and redirection of force, Rin knows enough to protect herself from an average, mundane opponent. She can fend off a common criminal, but someone with a specialty in unarmed fighting would have an undeniable advantage over her.
Tools of the Trade: Rin almost always carries tools and a ritual dagger with her.
As someone who likes to be ready for anything, Rin usually carries tools on her to augment her Magecraft. This includes a cigarette lighter, extra water bottle, and at least a jewel or two in pocket for emergencies. She always has her Azoth knife with her; a ritual dagger and focus that has enough energy stored in it for a single offensive blast. It's not as powerful as her jewels, but "rechargable, and also not turning into a pile of slag when used" are pretty useful benefits.
Magecraft (Utility): Rin can detect spiritual beings and (ab)use Magecraft for minor mundane tasks.
Rin can use Magecraft for low-level tasks, too. Some uses include repairing a broken pane of glass, stirring a coffee cup without touching it, or detecting magical energy in others. Detection can give her an idea of what she's dealing with in terms of how far beyond human baseline a given being is. Thes low-level effects are rarely useful by themselves in combat, but they can be useful in other ways.
Magecraft (Combat): Rin can attack, form barriers, and wield elements using Magecraft.
While Magecraft is usually the domain of scholars, there are things it does well in a combat context. As a rare "Average One" skilled in using all her world's base elements, Rin can manipulate air, fire, water, and earth as a defense or a weapon. By shaping the four elements, she can form barriers or shields. Alternatively, she can hurl them or otherwise shape them as weapons, lashing at her foes with serpentine streamers of fire, dagger-like shards of ice, slashing winds, or stinging sandstorms. Although her skill is good with all four base elements, fire is her go-to, as her late father's specialty.
Magecraft (Bounded Fields): Rin can set up defensive wards that can alert, conceal, or protect.
With time and resources Rin can weave ambient energy around an area, establishing a protective ward. The size of an area can run from a closet to a modest department store, but larger areas require more concentration, resources, and time to establish. These warded areas can't be packed up and moved once they're set, and they allow Rin to alter the area in various ways. Concealment is the most common, hiding an area from audio or visual detection. She can also key an area to alert her if anyone crosses the boundary line. Rin can also shield an area to protect the people inside it, though the more Rin is trying to shield, the more energy it costs her. Bounded fields aren't invulnerable, though, and they can be disrupted by attacking the drawn sigils that define their area.
Magecraft (Specialty): Rin can use jewels to boost her attack and defense Magecraft.
The Tohsaka family's specialty Magecraft is the transfer and storage of energy. Jewels are the best medium for this, and Rin uses them like a kind of energy battery. After infusing her own essence into them for years, each jewel serves as a single-use spell, with power orders of magnitude greater than what Rin can do with her own natural reserves. Each jewel is bound to the element of air, fire, water, or earth. Of particular note is that the jewels can be used by anybody, requiring no special training. Lastly, as heiress to a long-standing magus family, Rin stores generations of Tohsaka research in a tattoo-like crest on her left arm. It lets her access all of her family's knowledge and spells. As a handy bonus, if she's badly wounded, it'll act as temporary life support.
Complications
Perfectionist: Rin wastes effort, resources, and time to get her plans just so.
Driven by professional pride, Rin can go overboard with her planning. Whether preparing an important ritual or cooking an intricate meal, she will meticulously check and double-check her preparations. If things lean a bit sideways she'll pour absurd effort, resources, and time to set things back on track. In a fight it could mean giving up an advantage, dropping her guard, or simply wasting resources.
Tech Averse: Rin is suspicious towards technology and prefers Magecraft.
There is a culture among magi of open suspicion regarding technology. Rin is more accepting than most, but she still doesn't trust advanced technology, expecting the worst and usually getting it. She's not stupid, but she does have an inherently negative bias. It doesn't matter to her that it might put her at a disadvantage. It doesn't matter to her if Magecraft is sometimes less convenient. She'll go out of her way to shortcut around relying on technology.
Inexperienced: Rin can be derailed or shocked off her guard easily in battle.
Psychologically, Rin isn't cut out for battle. There are times when this is obvious, easily derailed by plans not going her way, or shocked out of her plans by the sight of atrocities. She isn't mentally prepared to address chaos and death on a wide scale, and she often needs some adjustment time to cope; time that isn't always available in the heat of battle.
Sister Dearest: Rin will drop everything if her sister's in trouble, no matter the cost.
The Tohsaka sisters are inseparable. Rin sees her sister as all she has left in the world, and considers herself personally responsible for Sakura's protection. She'll always prioritise Sakura's safety over anything else, even her own safety, or obligations to allies. In combat she trusts her sister to hold her own, but Rin will drop what she's doing to intervene if she feels Sakura is genuinely in danger. Unfortunately this can also mean leaving allies high and dry, or giving up an advantageous position.
Pesky Moral Compass: Rin has morals and sometimes they get her into trouble.
Most magi are morally bankrupt monsters, but not Rin. Her morals are something she absolutely won't budge on. If she feels they're crossed, she'll get involved even at risk of danger to herself or others. If she witnesses or hears of injustice, she'll get involved even if it's none of her business. Especially if it's none of her business. Personal risk matters less to her than doing the right thing.