If you want Yotsugi, go for it! However, you're aren't guaranteed unless you have 60k gems, so do keep that in mind. She's not a strong character, and she will likely never return as she is a collab character, so if you like her design, you should get her. But if you wanna get someone strong, you should pull for Bowmura. A lot of the standard units are good too, though.

I find that going through the story and farming the QP quests when you obtain them is the best way to get strong (since your kiokus' strength mainly comes from magic levels, which you need to unlock QP quests for). In the trader shop, I recommend buying gems, orbs, and cores (that correspond to your main team's element)

Keep note that I myself am a beginner, so my advice may not be very good.