Go with Misandra, she’ll pair well with Athena. On average Misandra will deal more damage than Richard and Thorne with her skill: [Research Math] Calculating Misandra’s Chain strike average damage
Richard is one of the best blue tanks in the game IMO – particularly when emblems work together with his attack down debuff – but you don’t get much rewards out of defending in this game. Unless your alliance wants to use blue tanks for wars, you already have Guinevere, and thus Richard is not a priority over Misandra.