( Your character and CasterCollapse )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!)