My noob opinions:

Garrett is an offense based tank. His trust bonus and skill give attack. On max level, he has lower base defense than Leeanne, whose skill also boosts defense. Because of that I would go with Leeanne for now.

Marr is great from what I've heard, but I would level a conventional healer (or maybe even two) first for the big single target heal and higher range. Iris has high healing output during her skill, while Fedora is more flexible with the increased range. I'd go with Iris first, especially since you just got a second one to CR/SU her later (keep in mind that her skill changes after CC, so you will have to CC the dupe for the SU).

As for Lilia, I would give her some xp (level 25-30 or so), but get the core team up and running before leveling her further.