Ilya is doesn't need her own route. She knows that everyone considers her best girl anyway.
I'm going to say she is best girl, and now I'm going to leave before the storm comes
Rin is objectively the best.
Why would you like an albino midget slut better?
Maybe she sucks at onee-chaning but she'd eventually become amazing at ara~ara~.
That's if she was allowed to, fuck you Nasu.
Secondary girls are literally better than any main heroine.
getting impaled from behind is a tohsaka family tradition after all.
The entire point of Illya's character arc is that Shirou was never the onii-chan, she was the onee-chan the whole time. She was the one lecturing him and showing him guidance when they were having fun in the park.
She's not very little-sister-like at all in Fate/Stay Night.