It's best if i don't tell you shit user. However, watching or reading the original Fate Stay/Night would give you a really good idea of her character. Granted if you've watched Fate Zero you know her true relevance. To a lesser degree Unlimited Bladeworks could redpill you alone. Unless you like lolis and yuri bait stay far away from Prisma Illya unless of course you want to be made into a lolicon ha ha