I'll be able to claim the constellation from To Temper Thyself but I still can't decide which one to choose. Which one are you choosing?

I'm picking up Shenhe, but with your lineup I'd be tempted to pick up Mona or Klee to try building some flavor of hex vape team

Pick whoever you like/use the most because guess what you get two more after this one.

best option is the character(s) you like playing most, other than that here is my priority list based on your characters

1. i'd say Klee probably has the best con of the available options, considering she's also a fair bit stronger at baseline compared to other characters here thanks to her recent buffs

2. next i'd say shenhe and nilou have good Quality of Life cons that help extend their relevant skills/buffs' uptime to support their teams. my order would be: shenhe C2 > nilou C1

3. Ganyu gets a small but unconditional boost of damage and energy, pretty nice

4. Yae C1 allows her to use her burst more often which is nice but depends on the team you are playing, since using burst combined with her already long field time from setting up her elemental skill eats into the field time of your carry or eats up the uptime of buffs/debuffs

5. kokomi, cool damage buff for on-field DPS Kokomi, but outside of that it doesn't do a whole lot except apply more hydro when on-fielding her

6. yoimiya, burst uptime extension is basically useless, the ATK buff is nice but small and conditional (needs to burst, which takes up field time and doesn't do a whole lot by itself + can't even get the buff in content with just one enemy like stygian + in multi-target, where she is already bad, it doesn't help her untill she has already killed one enemy).

7. any standard character to get closer to the chance of obtaining a masterless stella fortuna, but considering you can also get their cons from losing 50/50 or pulling on standard banner i'd consider picking the above options first before these

8. ayaka, only value her C1 gives is that it gets you one step closer to C2, which still costs \~160 wishes

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for my account i'll be choosing baizhu's C1

c2 shenhe if you have skirk, effie. c1 yae if you have the bina+ineffa wheelchair. nilou and kokomi if you run lunar bloom. klee and mona if you have durin.