Theo von Ordelia's Links
He cares deeply for his little sister despite their parents' clear favoritism. Especially as the twins felt it was their responsibility to protect her while their parents fought.
Theo often tutors her in different subjects she struggles with. Especially once she becomes the heir, he realizes that for her to be able to act properly as the heir she would need him.
She grew up too sheltered with little need to grow meaning that she stumbled and failed repeatedly as she became the heir. Without Theo, she would have made many grave errors that would have negatively impacted their house.