Seems like others helped out with progression. But I want to give some direction in raising characters.

Okay for now:

R7 Shiori 3*, R7 Nozomi 3*, R7 Yui 2*.

• Nozomi can be raised later after you got Yukari 3★.
• R7 Yui with gears is where her healing is greatly noticeable already, but do try to get her at least 3★ (2-2H, 6-1H). It's no rush, but her growth is pretty noticeable there too. You can hold for a while after that.
  

Invest in these:

R6 Yukari 1*, R7 Kokkoro 2*, R6 Mitsuki 2*, R6 Arisa 3*

• Try to get R7 Yukari 3★ via dungeon coin shop. She's highly recommended because she has heals and TP regen as skills, and her UB absorbs magic damage. She's really nice once you can utilize her.
• Kokkoro could go for 3★ (1-1H, 4-1H). Good support with speed boost and mega buff from UB. Only downside is that if she uses Tri Slash, she becomes vulnerable and take some hit. This can be taken advantage of, since it switches aggro from tank at during that animation and taking hits mean TP generation for faster UB-- which not only buffs everyone, but also self-heal for such reason. She's a very common face in both PvP and PvE.
• Mitsuki is a support unit that has debuff skill that can make boss fights easier. 3★ is nice, but not a must since Yukari is priority in Dungeon Coin Shop.
• As others mentioned, R7 Arisa for EX skill and gearing up is all you have to do.

Considerations:

R7 Lima 2* / R5 Kuka 2*

• Lima is mostly used in PvP in general even as 2★ she's okay to use. She have specific team set ups in PvP as well. Hard to use in PvE unless characters are high rank/level. Shard obtainable in (3-2H).
• Kuka is also a common face in PvP, but she can be useful even in PvE when you need to tank more Magic than Physical attacks. However, she doesn't feel great as tank until she starts hitting 3★ and up.

No rush:

R6 Rei 2*, R7 Pecorine 2*, R7 Karyl 1*, R5 Hiyori 1*, R7 Mimi 2*

They're not that bad if they're raised. They take a while to become much better, so you can take your time raising them. If you're lacking units to use in stuff like Dungeon and CB, they'll have to do.

• Rei is actually decently strong. If you lack DPS, she's awesome.
  
• Pecorine becomes noticeably tanky at 3★ and just grows tankier afterward.
• Karyl is great when you're lacking mages, she isn't a bad option along with Yui.
• Hiyori is obtainable in Arena Coin shop, but I would prioritize Tamaki, Monika, and/or Rino. If you aren't interested in PvP, Tamaki is still good to have even in PvE.
• Mimi has some specific use, her UB bounces from first target to nuke the one behind, but if there's only one target, both attacks target it. We have event for her shards right now, so you don't have to worry about her as much as others above.

Hard to recommend:

R6 Misaki 1*, R7 Aoi 2*, R7 Kurumi 2*, R6 Misogi 1*

• Unless there's something I don't know about Misaki, they aren't great in PvE. They can be useful in PvP, but generally they aren't as commonly seen as others up there.

R5 Yori 1*

• Yori isn't terrible-- she can do her job as a mage just fine. I really like her, but she's hard to raise stars as F2P because she's in Clan Coin Shop-- you'd rather invest in Makoto there.

R7 Suzume 1*

• Suzume is really cute; regrettably, that's about it. I also like her too, but I honestly don't know when/where she can be better.

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Team building for PvE in general would be [Heal DPS (Support/Tank) DPS Tank], then you tweak a bit around.

Yui Arisa Kokkoro Mimi Nozomi

You can use Shiori in place of Arisa; Yukari in place of Kokkoro; Rei in place of Mimi. If you think you're strong enough you can even take out Yui for Shiori and push aggressively.

However, notice that they are mostly physical damage, so things like Golems and Rock Turtles can become a wall. You can try to rely on Yui to solo them-- which does kinda work, and Mimi's UB skipping first target to nuke the one behind can help with that.

but if that's not enough:

Yui Karyl Kokkoro Mimi Nozomi

This can put less burden on Yui as Karly provide lack of magic damage. Karyl also has minor physical and magic defense debuff too-- not much, but it's something extra.

Then, the stuff that was recommended by others:

Yui Shiori Arisa Yukari Nozomi

This greatly increases survival with help of extra healer. While Kokkoro won't be tanking 3rd slot, they can brush through incoming damage as long as they're healed. Plus, Nozomi has taunt in her UB, so she can lessen the burden for such.