Hi! Please never be sorry about anything you dump in my retrospring, it's here for that reason~

Hmm, yeah Leo is hard to embarrass, he's pretty shameless in some aspects, which is especially fun to contrast with easily-flustered characters (in canon, it's sooo adorable with Tsukasa, the "Leo showing his tummy in submission bit" in Requiem never fails to make me squee).

I don't know if there's someone in particular who can makes him flustered more easily, at least not that i've thought of.... my biased shipper self wants to answer Eichi in the specific context of sex and Eichi being intentionally embarrassing and cringe and Leo not being able NOT to react to that, but embarrassed in a shy way? My best guess is, same as you, both during and outside of sex, during a moment that truly shows the other character's feelings and the depth of their intensity. There's definitely some people he would be LESS shy around I think (in my mind, Madara, or characters like Tsumugi) but I do struggle to see him too easily flustered around people! At least in canon; fic-wise i obviously enjoy making him flustered in various ways~

With Izumi in particular i think Leo isn't easily flustered, because he has shown time and time again he's able to profess his love for Izumi in front of others and in very clear ways; there's a chance he could become flustered if Izumi responded to that in a vulnerable, genuine fashion, but even then I feel like Leo wouldn't be flustered because he would just accept it? Since he understands these feelings and shares them. Meanwhile with someone like Eichi, they're obviously fond of each other but tenderness/vulnerability is definitely not a thing they expect to share. So lots of potential here <3

I hope my answer makes sense!