That's because Casimir is a rather difficult man who doesn't trust easily. To make him fall for your MC she basically has to spend with him as much time as possible, support his decisions, help him fight his enemies, be fearless, loyal and honest :) Casimir is also sensitive about his social status so be mindful of that. Remember that Oriana can help Aria determine the progression of her relationship with him.
I'm not planning to make a walkthrough - I want you guys to explore the options and see for yourself what works for different characters!