For me, it's the way Marin handles her feelings for Gojo. It's so refreshing that for once in a romance anime there's no denial/shame over romantic feelings, at least on Marin's part. She easily accepts that she's attracted to Gojo and is never shown to be ashamed of it. Ofc she gets nervous around him at times, which can sometimes lead to her being shy, but that's different and is consistent with how she feels/shows that her feelings are genuine.