i know the typical aceyuu schtick is like, tsundere ride or die and isekai protag who doesn't take shit, WHICH IS GOOD I CAN'T GET ENOUGH OF IT !! I LOVE ACEYUU (and adeuceyuu!!)
but ahh, Dinah & Ace is a little different. I like to think Ace is "aware" of the loops too - via super cool lore significance (WHAT IS HIS UNIQUE MAGIC. THE FIRST PERSON YUU MEETS ASIDE FROM GRIM. TRAITOR ACE THEORY. WHAT IS UP WITH THIS GUY) and just knowing Dinah the longest out of all the characters (besides Grim, of course. Grim also get the special treatment here).
He's not affected like Dinah in the sense of "knowing the motions and going through them tiredly"—he's the wildcard of Heartslabyul—he's just, really in touch with Dinah and his mannerisms.
Lingers in the mirror chamber to watch Dinah and Grim during orientation.
Regards Dinah with a hint of familiarity, doesn’t make fun of him for not knowing the Great Seven (alternatively, finds it normal that Dinah seems to pick up on the legends easily, whereas everyone else regards it suspiciously.)
When they go out to walk around Sage Island, Ace asks Cater and Ruggie for new restaurant and hangout recommendations to offer Dinah some variety (knowing how much the prefect’s face lights up at something new).
Tries to convince his family for him to come home later/come back early during Book 4 (And in later loops, he struggles with trying to ask Dinah if he wants to come home with him. It’s only been a couple months since they’ve met, but Ace feels like they’ve known each other for years.)
picks up on Dinah’s headaches and mannerisms faster, always quick to point out Dinah mimicking others since it’s different every time. (When Dinah mimics him, his chest swells with pride.)
Has the strangest feeling Dinah’s going to do something stupid at the beginning of Book 6, he wakes up faster than Deuce and has to be held down so he doesn’t go after his friends.
[There’s also the theory that Ace rigged it so Yuu could attend Vil’s hometown event. In the context of Dinah Yuu, Ace knows how excited he gets at the prospect of a new environment.]
He gets a little jealous when Dinah pursues close friendships with the Overblot victims or upperclassmen, but he holds his tongue when he sees the excited smile gracing Dinah's usual neutral expression. Besides, he's still one of Dinah's best friends, the prefect will be back soon enough to play card games and laugh with him.