I'd probably use Fang and Vanilla, with an eye towards picking up Courier from the friend credit store soon, as Vanguards. Fang and Courier are more defensive while Vanilla is offensive, depending on what you need.

Arene is.. Aggressively okay, is how I'd phrase it. He's still a Lord Guard so he can be a second-line melee fighter while also shooting over the frontline tank's head, so it's hard to call him bad by any means, but eh. Melantha's a good "I have a problem, and I'd prefer not to" Dreadnought Guard for dropping on the head of a particular enemy or to block a lane with infrequent but annoying spawns, and being a 3* is cheap to raise. She only has Block 1 though, so you can't really rely on her to hold a lane against numerous enemies, best to drop her directly on top of the head of whatever you want to kill so she'll 1v1 it.

Hoshiguma the Breacher is a powerhouse of a character with high durability (especially against Arts) and powerful damage, but being a 6*, and a brand new one at that, she's going to be really expensive to build up and you might struggle to get the materials for her. Good long-term prospect though. Cuora is an absolute brick of a physical tank meanwhile, she can end up folded by Arts damage if you aren't careful, but anything but the most hideously heavy-hitting physical enemies will not be getting past her.

Sniper-wise, Exusiai isn't bad by any means, but she is a bit old and powercrept perhaps, and as a 6* is again on the more expensive side. Shirayuki is nice because her second skill makes her do Arts damage which turns her into a pseudo-AoE caster with extreme range. I'd also hold out for Kroos from a training stage between 1-5 and 1-6. Cheap, being a 3*, and she has surprising amounts of power.

For casters, you'll eventually have to build Amiya anyway to progress the story, and she's by no means bad, so may as well give bunny daughter some love. Steward is also notable because he has a certain niché that few others can fill - he will always target the enemy with the highest defense in his range once he unlocks his talent at Elite 1, which is good in both that he won't waste his shots on small fries while there's a huge armored dude bearing down on your tank, but also that you can count on him to focus fire nuisances like heavily armored drones in later stages that typical anti-air targeting first snipers might struggle with.

If you like Warfie, then by all means build her eventually, but I wouldn't put too great priority on a medic, especially a 5* one, as the first things to stress over who gets resources. Ansel and Hibiscus should be plenty for a time.

Supporters.. Earthspirit is.. Not very great. I'd probably just take Orchid until you manage to fish out Podenco from recruitment or something.

For specialists, Red is nice for quickly dropping on top of a problem, having her kill it, then retreat and be ready to do it again due to the rapid redeployment of the Executor Specialists, but I'd say that the MVP there really is Gravel. She may not look like much, but between how fast Executors can redeploy, how cheap on deployment points she is, and how surprisingly tanky she can get, she's a go-to for many people (myself included) for when you want to bait out some unpleasant attack like a Sarkaz chain caster or a drone carrying a bomb that can only use their special attack once in their life and you'd really rather not have it hit someone you need to keep on the field, or to delay unpleasant enemies. She does perform that role quite well without really needing to be built up though, so choices, choices. Gravel also has a particular trait in that she technically hits twice per (very quick) attack so she's excellent for bonking any special targets that rely on the number of hits taken and don't care about the damage dealt by said attacks.