Eyyyyyy, another FGOer adds another layer of hell to their lives.
Imo, you can almost never go wrong with who to raise. Just look them up on GP and pay attention to their listed archetypes and skill sets. Some of them are really similar or even downright the same minus the stat and skill values.
For example, both Fang and Scavenger have the same Skill 1. When their SP gauge fills up, they give you a sum of DP to use. Scavenger just gives more than Fang, but has higher SP cost. But looking at their stats, Fang is more defensively oriented, so she’s good for tanking early waves whereas Scavenger’s higher ATK is better for quickly killing weak enemies at the start.
And unlike FGO, where low rarities already have lower stats than high rarity counterparts, Operators at the same promotion level (the equivalent of Ascension) are pretty much equals (or if not, the differences in performance are fairly slight and almost insignificant).
For example, you’ll see little difference between a level 30 Kroos, a level 30 Jessica, and a level 30 Exusiai (6* Sniper of the same archetype and similar Skill 1 to Kroos and Jessica). Their starting stats are skewed more towards higher rarity, but not so much that the difference becomes immediately apparent during gameplay. In the beginning, you could easily replace Exusiai with Kroos of a similar level and you’d barely notice the difference.
Stat differences start to become more apparent as you level them. Even at E0 promotion (the base promotion level), Jessica has a higher level cap than Kroos, and Exusiai’s is even higher. Still, the differences aren’t so great that you’d immediately stop using Kroos.
Promoting them to E1 (resetting levels BUT NOT the stats), means Exusiai’s stats can shoot even higher than theirs because her new stat cap is still higher than Jessica’s or Kroos’s new caps.
Furthermore, 4*-6* Operators can be further promoted to E2. So Jessica and Exusiai’s stats shoot up further even after Kroos has hit her final level cap at E1.
So TLDR, don’t worry about leveling up the wrong Operator. Tons of videos and YTers out there showcase low rarity clears with varying tactics. It’s one of the rare games where you can literally play however you want (provided you don’t decide to only raise one class and think you can win everything).
Just as how FGO’s rule of thumb is to raise one ST and one AoE, rule of thumb is to raise 2 each of Vanguards, Guards, Defenders, Medics, and Casters. Those ten will form the core of most any team, especially at the beginning. The remaining two can be just about anything or anyone you have on hand you think could help.
For straight up recommendations from your current roster...
Scavenger and Fang - Gives DP whenever their SP fills up. They’ll be the backbone of your team for days and weeks to come. If you go to Store > Credit Store, tap on Operator Progress at the bottom of the screen. Once you spend a certain number of Credits listed there, you unlock the appropriate Operator in the shop next time it resets. Courier is a more defensive-oriented operator, so an upgrade to Fang (though with a higher DP cost so it’s up to you who to use) or a substitute for Scavenger if you find Scavenger dying too quickly (though his ATK is lower than Scavenger’s by comparison).
Plume - her Trait (a passive skill) is that she gains 1DP per kill. But unlike Scavenger or Fang, she can only Block one enemy at a time. So placement needs to be deliberate or else you risk someone slipping through at an early stage.
Melantha - If you look up her stats you’ll find that her HP and ATK are among the highest in the game despite being a 4*. She hits hard and can take hits.
Kroos and Jessica - Their Skill 1 is simple. After each attack, they charge their SP. Once full, their next hit harder. Then repeat. Simple and very effective. They have the ‘Anti-Air Sniper’ Archetype, meaning if there is a aerial drone in their range, they WILL target it first. Which is good, because other ranged units might accidentally let those slip past if they’re attacking something else.
Rangers and Adnachiel - slightly more niche than the two above. Rangers deals extra 50% damage to aerial drones (needs to be level 30 to unlock this Talent), and when Adnachiel is promoted to E1, his Talent lets him focus on Ranged enemies first, great for taking out enemy snipers and casters moving among groups of melee enemies.
Shirayuki - an AoE Sniper. Lower attack speed, but great coverage. Her first skill increases her range massively. Her second skill, unlocked at E1, is where she shines, however. It lets her deal Arts type damage for the duration of her skill. And in this game, Arts damage pierces through DEF. So despite normally dealing physical damage, she can also switch to Arts damage for the duration of the skill to help take out any high DEF enemies in her range.
Gavial - You can get her in the Credit store the same way as Courier. The two of them will alternate appearing in the shop each day provided you’ve spent the required amount of credits already.
Ansel - I know you don’t have Ansel, but you’ll eventually get Ansel via Recruitment. I prefer Ansel over Hibiscus because Ansel’s Skill 1 provides one extra block of range for a little more versatility in placement. The Talent unlocked upon promotion to E1 gives a small chance of healing more than one operator at a time (low at 7%, but it’s there.)
Hoshiguma - definitely level her. She hits back pretty hard, unlike most other Defenders, while still able to take a beating. Since you only have 1 Guard at the moment, she can act as a replacement for both Blocking and attacking enemies.
Matterhorn - different from most ‘Normal Defender’ archetypes because he not only possesses some Resist stats (lowers Arts/magical damage) but his skills raise his Max HP, rather than raising his DEF stat. Not bad, but if you need extra tankiness when high ATK enemies come calling you may need someone else.
Beagle - Like Ansel, you’re sure to get her from Recruitment. More straightforward than Hoshi or Matterhorn as she is a straight up tank with a decent DEF buff for her first skill.
Dur-Nar - Once you get all 6 copies of Courier and Gavial from the Credit store, Dur-nar will take their place. Hoshiguma’s Skill 1 gives her both an ATK and DEF buff. Dur-nar’s give her only ATK. But her second Skill, unlocked at E1, lets her deal ARTS damage. So though she’s not as tanky, she’s a decent counter to high DEF low RES enemies. To make things better, she’ll attack up to 3 enemies all at once while the skill is up, provided they are in her attack range.
Amiya - A great ST Caster, enough said. And as mentioned above, Arts damage pierces through DEF, so she’s great for taking out High DEF enemies.
Gitano and Lava - Lava is another one you’re sure to get from Recruitment. Both she and Gitano are AoE Casters. Lower ATK than ST Casters, but made up for by being AoE. You’ll need at least one for clearing groups of enemies before they overwhelm your defenses. Two is preferable should you need it.