That is the face of a man who will not be NTR'd.
I understand why Tiara's so worried about her situation, particularly with the universe seemingly conspiring to make the event CGs happen even if she tries to avoid them, but I wish it would occur to her that she can tell Aqua what she's concerned about - poor guy's practically begged her to do it. It could even be a tactful "look, she's hitting the storm of puberty, and you're the hottest guy in any room... of course I'm worried she'll crush on you and you'll accidentally break her heart without meaning to" instead of going straight to "YOU ARE MY MAN AND I'M WORRIED THIS AISHIRA BITCH WILL TURN INTO A THIEVING WHORE AND TAKE YOU AWAY FROM ME!" (Honestly, it would probably have been best if she'd just jokingly said "oh, she sent a secret message saying I should be worried about some... Aishira? stealing your heart from me." after reading the magic letter from the first heroine, and given Aqua the chance to think about the fact he might want to keep a bit more distance from this teenage girl he works closely with.)
I don't know if Tiara knows exactly how lucky she is that Acqua didn't watch her, who won't tell him straight-up what's obviously worrying her, open right the fuck up to Keith about it the first time they met. (Like us readers got to watch.) Right now, he's just mad at Keith. If he knew that piece of it all, he might be (a wee bit justifiably) irritated at her too.
...fuckin' first love, where everybody's so happy to be with the other person they feel like they can't open up about anything too deep for fear of scaring the other person off. Much as I can kibitz about what these people are doing wrong, I can't say it's an inaccurate portrayal.
What really bothers me about this setup is that Acqua staying with Tiara could potentially piss off both the sea fairies (if Aishira falls for him and he rebuffs her too harshly) and the earth fairies (because Keith), while going with the intended heroine Aishira and leaving Tiara for Keith to pick up on the rebound keeps both elements of fairies happy. In fact, an ending where Acqua got with Aishira and Keith took Acqua's fiance would be almost perfectly parallel to the way Hartknights got the last game's heroine and his fiance ended up with an even dreamier capture target...