Here's the visual impression that Caster gives, depending on your character's level of perception:
♦ General:
Height: 163 cm
Weight: 51 kg
Three Sizes (B, W, H): 82, 57, 84 (with the Japanese metric system)
Voice:
[1] [2] [3] --Beginner: Without any training at all, it's easy to see that:
♠ Caster infuses each of her movements with grace and elegance. Of course, if she's wearing that robe, it'll be difficult to see until she extends her hand from the depths of black and purple, but her walk and voice will still retain those qualities.
--Intermediate: Is your character wanting to observe Caster? Or perhaps he/she just happens to be particularly perceptive. In that case, you don't have to notice everything, but if you're looking out for it, you can notice that:
♠ Caster's confidence and refined movement simply scream proper breeding and there is -- just maybe -- pride or royalty in the way that she carries herself without ever slouching or slumping her shoulders. Her hands are also delicate, clearly unaccustomed to hard work and carrying heavy things. Not only so, but unless she has a reason for it, she'll walk right down the street without glancing at the people that she passes. It's another dead giveaway of the same, unworldly feel that a princess gives, especially one who's never had a need to observe her subjects.
♠ Caster's clothing choices, while still in keeping with her dignified conduct, are the faintest bit old-fashioned. She's generally a self-respecting woman, who doesn't like to show off more skin than necessary, and she wears purple lipstick to match with her wardrobe in indication of having an eye for color coordination.
♠ If you can catch her without the hood of her robe, it's easy to see that she's a little more emotional than she appears. Look into her eyes when you insult her, and you'll see them flash even though there's already a calm retort on her tongue.
♠ Whenever Caster sees a cute, innocent girl, her eyes soften, and there is, perhaps, an added touch of distance. Conversely, when there's an arrogant man in the room, her eyes harden, and her smiles have a harder time reaching her eyes. Feel free to draw any conclusions you like.
♠ Why does she wear that robe? Maybe she wears it to be seen as an equal, regardless of her gender. However, she's proud of being a female, and she'll expose it through her penchant for feminine clothing and wearing lipstick. Combine it with the fact that she conceals her curves and occasionally hides half of her face, and you'll see that she wants any man to know she, a woman, can most certainly match your wits -- perhaps even best it.
--Expert: Are you keen enough to be on Sherlock Holmes's level? Then you are at complete liberty to notice that:
♠ Caster's steps are just a little bit heavier. It isn't the least bit noticeable nor detracting from her gentility, but if you notice tiny details like the impact and weight of her steps against the floor, you'll see the slightest difference from a regular female's walk. Done subconsciously every time you see her, it would seems like there's always a guilt or a burden weighing down on her mind.
♠ As the days pass, people normally gain or lose a little in weight, whether it's the fraction of a pound or more. Now, Caster? She's always exactly 51 kg. Her measurements don't change with time, no matter how much or how little she eats or sweats. The same applies to her height.
--Psychics/Magical/Supernatural
♠ Caster has a strong resistance to hypnotism, binding, coercion, and the like. Essentially, trying to read her mind or control her body via magical means isn't likely to work. However, I won't go so far as to say that it's impossible (but it is exceptionally difficult), so if your character happens to be extremely powerful in this field...just let me know, and perhaps we can discuss a plot. ♥
♠ Being a heroic spirit, she'll feel inhuman to any of your characters that can sense that kind of thing. Although, even if you don't have that capability, her elf-like ears might certainly be a good tip-off.
--Other
♠ When it comes to hugging/flirting/being creepy/attacking/mentioning something in particular/etc., go for it! Just expect Caster to react accordingly. However, when it comes to anything severe (i.e. maiming or killing), I'll probably be open to the idea, but please do contact me first! It'd be great if we can work out a plot. Although, do keep in mind that it's difficult for her to meet her demise, to say the least.
♠ As for stalking her, it's...actually rather difficult, since she'll either have a boundary field around her territory or, if you're using a power, she'll take notice. But! I'm more than willing to work this out if you have a plot in mind.
♠ Backtagging and, unless it's filtered specifically, threadjacking is all good with me!
♠ I rather that you don't fourth-wall her, but I'm fine with it happening to her true identity (detailed a little better below!).
Now, regarding Caster affecting your character with her abilities:
As a top-class magus and heroic spirit, Caster wields a vast amount of magic, but, as a general rule, she won't use her spells on anyone without the other mun's explicit permission first. However, there is one ability that she'd like to use quite often for observational purposes -- namely, the one labeled in that long, long ability list as "Magic Sensory".
Like its name suggests, Caster can sense your level of magic, the strength of your resistance, or even your potential for it. This includes the supernatural and Core-related abilities. In Fate/Stay Night, true magic is considered difficult to achieve. In other words, she senses the supernatural, and while she can't discern the exact nature of the ability, she can discern its strength, as well as how it differentiates from another's ability. (If you're familiar with the Nasuverse, then this is in terms of D&D style of ranking strength, agility, mana, luck, and endurance.)
Anywhoo, I have what is hopefully a handy-dandy text box below, and if you don't want her taking notice for whatever reason, consider it done! :3
Also, her real identity is partially connected to her abilities, in which the more people that recognize who she is, the stronger they are. So, included in the textbox is a question about whether your character would know/heard about she is. I've avoided writing it out here, in case anyone wants to watch/read her canon without prior knowledge, but it's in the text box question, so you may want to avoid the last one if you want to continue being unspoiled.
(So! As a side note, if your character ever finds out her true name, you are absolutely free to fourth-wall her about it!)
Can Caster sense your magic level, resistance strength, or potential?:
If so, what is its level/strength/aptitude?
Does your character know about the Greek legend of Medea?
Anything else? (Chances are, if your character isn't human or has a contract with a familiar, she'll be able to tell.)