Wu before Gretel. He will nearly double your damage against titans. He’s also pretty useful on maps, quests, or raids, 'tho he is pretty squishy, even maxed. Especially for raids where you can’t use mana pots. (If he is surviving against titans now, you aren’t fighting big enough titans And it is worth preparing for that.)
If he really is surviving, Gretel has real potential for mana control. I’ve only started to level mine up, and while I believe Hansel is noticably better than she is, she’s probably going to be worth it too. BUT … she will be your third yellow. Del and Wu are SOOOOO useful that you won’t want to leave them behind very often!
I’d suggest Caedmon over Melendor 'cuz Delilah is such awesome healer, and a hitter/debuf lets you go out with three hitters, Del, and Wu.
Eventually Melendor too, though.
Scarlett before Gormek – She’s a glass cannon and needs all the HP she can get, and you already have one of the ramming pulverizer triplets ascended.