He's straightforward, cool-headed and logical. Sooo cool-headed that he doesn't realize he's been showing his interest and feelings for the FL the whole time while justifying that it's because FL is his sister's friend but when he does realize his feelings, he basically goes "Yes, i like her, i want her, I want to be with her, I want to protect her" and he courts her.
Not this whole awkward blushing and stuttering in the Manhwa. In novel confidence is what moves that the FL to step away from her attachment to the original story and start cherishing herself more.
The FL because of her trauma and thinking ML belongs to heroine, lies to herself about the interest he is showing but because he doesn't talk around it when he does confess she can't help but want accept.
His whole personality and his sister are strongly tied to the character development of the FL which is why i think it's sad how they watered it down here to the typical cliche moments.
Eve and his brother essentially start competing and making deals to spend time with the FL (โงโโฆ)

NOOOO THEY RUINED THE GIFT MOMENT
Was so cute in novel
I know they've changed a lot of things already, including ML's personality but โ( T๏นT )โ
Read if you're can survive with ruining your webtoon experience:
Novel, he went to get a present for his sister because it's something he ALWAYS does whenever he goes to another territory. Eve sulks when he doesn't bring at least one present. He went to get her the trendiest (also expensive) bracelet when the earrings caught his eyes and thought of the FL, he goes through his thought process and buys them. Which is what makes it so adorable when he gives her the earrings because he shows her that he was even thinking of her without being promoted to but now it feels like he was pushed to go buy them RIP my feels on the fluffy moments