Isarnia, without a doubt. In fact, you are just “Isarnia being leveled” away from having a top notch defensive team IMO. Both her & Richard are ‘decent’ (B) as a defensive BLUE, but you don’t need Richard’s tanky stamina benefit with Guin as your tank.
The best tank… make 'em weak w/ Isarnia… and three great damage dealers. You’ll defend many raids with that lineup.
Put it this way… If your lineup w/ Richard came up in a raid for me, I just may attack you because Richard doesn’t scare me… If you had Isarnia there instead, I’d be more likely to say, “Ehh… No thanks… SKIP”. lol