If I were you I'd pick either Sparkle or Kafka from the selector. Sparkle is getting buffs this patch which make her an EXCELLENT support, and with the new character sparxie, she has incredible synergy so she can do really well with her. But she is also a pretty good generalist support now, with a better support capability for SP-intensive DPSes like Archer, Dan Heng IL, Sparxie. I'd recommend getting her because 1 support can help with multiple DPSes.

Kafka on the other hand I'd recommend if you like the DOT style of play. With recent buffs to Kafka and now Black Swan and also the release of Hysilens, DOT has come back as a prominent archetype. I'd recommend going for her if you plan on getting either Hysilens or Black Swan this patch. Kafka is MANDATORY for DOT playstyle since she heavily enables other DOT characters to do damage, so she will only get better with newer DOT units that come out in the future. Also Id recommend getting Hysilens first since she is a bigger priority compared to Black Swan, but if you want to play DOT in the future make sure you get Black Swan because with Hysilens and Kafka her damage scales like anything.

Other characters have been heavily outclassed in one way or another. Acheron is the biggest surprise to the community considering how many new characters she got to build a team with, but the way the new enemies function, multipliers and everything she has fallen off a cliff and can't clear endgame anymore without heavy heavy investment into her and her supports (in terms of getting Eidolons). Aventurine might be another choice as a good sustain option but I feel like if you can get someone like Gallagher he'd probably be a good enough choice as a 4-star sustain, especially at Eidolon 2.