Re: question
Well, obviously he doesn't get jealous in the traditional way, as in... this person is sleeping/married/in love and/or sleeping with someone else, so I'm jealous of them! since the people he gets involved with are usually taken, lol. And he himself is likely to have multiple partners, too. Even his comment to Adele that Richelieu "doesn't love her as he does" doesn't seem to come from a genuine romantic rivalry and more from the fact that Richie's a complete dickhead and really doesn't love Adele as Aramis does because of it, nevermind the fact that the musketeers all hate him, so of course he'd take a jab at the guy whenever he can. We never see him be jealous of the King, for instance, despite Anne displaying clear jealousy at points (so they could've easily approached it in the series, I mean).
I take it as coming less from an insecurity in terms of self-worth (he is perfectly confident, afterall, when he's not depressed) but more of a fear of abandonment. So Richelieu becomes a more equal romantic rival when he believes Adele chose him over Aramis himself (and his jealousy displayed there is a lot more like what you'd traditionally expect), and Alice is a threat despite being a lady Aramis would normally adore and Aramis having zero issue with Porthos/Widows (even being the reason they go to the church to pick ladies up in the first place), because her and Porthos's relationship means Porthos could leave. Anne is a non-issue in terms of that type of jealousy because Anne is the Queen and she can't "leave" in the sense the others can - she can't move away or anything like that. She's a stable presence and so it's irrelevant for them, and instead Aramis's bitterness at the King is more over the way he treats her, not because oh noes they're in love/he'll make Anne forget about me.
Therefore, he can get over jealousy if he's reassured abandonment isn't a problem. It's that, coupled with the fact that he actually knows her and likes her personally, what makes him not be jealous of Robin. It came up as jealousy when he was de-aged because he didn't have that reassurance regarding his relationship with Porthos that his older self has. It's the reason why it's a non-issue with Arthur, because Arthur being taken was part of the deal from the start, so it leans more towards how things are with Anne in that sense.
In general he's a big dumb loser when it comes to jealousy issues, and it makes him act childish and petty. If it got really bad I'm sure it could get pretty nasty, which of course makes me think I should event him to be like super jealous so he can be horrible, hm.
FOR YOU NOW, what are aspects of Isabela you'd like to develop that you haven't had the chance to touch upon, or that you think could be tricky to bring up in the game without canon circumstances/specific castmates?