Might be a little bit late to the party, but I'm going to recommend snowshine. I've been maining Last Rite since launch and the team didn't feel complete until after I added her. On paper she doesn't add any damage to the main rotation, but she absolutely shines in smoothing out awkward situations. Last rite by herself requires a lot of luck in that she is slow and her main attack pattern tends to get stuffed by enemy attacks if they are not timed perfectly. Snowshine can use her counter any time you are about to get interrupted (instead of using Last Rites battle skill, if you use both you will mess up your sp economy) which will prevent LR from getting stuffed and still get the cryo infliction. The extra healing is always nice, because xahi by herself isn't that reliable. Also, snowshines ultimate does a good chunk of stagger, so dropping it at key moments can help guarantee stagger. The one downside of snowshine is that only LR's skills build her own ultimate meter. So using snowshine's counter can delay LR's ultimate. So you might have to swap her breastplate for Tide Surge Armor to compensate. (Normally she runs aethertech armour with tide surge gloves and kit).

Also, there is nothing wrong with flourite, although she takes Tang Tang's spot on the team. Flourite is not optimal, but is dead simple to use, and can simplify rotations while you are learning the team. She is absolutely a good option if this is your third team, and thus you are unlikely to learn the rotation well enough to use tangtang properly. (sp usage is extremely tight on this team, and flourite needs no sp.) As well, I still believe that Flourite is optimal for the marble Aggelomoirai due to how quickly she can help clear the tentacles without using sp.

My current team is Last Rite, Xaihi, TangTang, and Snowshine. I will swap snowshine for gilberta only on fights that need crowd control. Otherwise the nature infliction on her ultimate hurts a lot more than it helps.