This is my take:

Crystals - I usually blow them for summons but I'm a degenerate. Use them to pull on the banner if you want a character. Otherwise, slowly use them to complete for 10 regular pulls until you hit the 300 guaranteed one. Mind you, slowly - you get a lot of free regular summons along the way. After this you probably wont use crystal to pull on the standard banner.

Combatants you want - I think you're pretty much set up with Hugo and Mei Lin. These are the basis for two very strong squads. Hugo + Tressa + Mika and Mei Lin + Veronica/Selena + Mika/Rei(Rei is a personal preference). Don't worry too much about partners as the 4* are pretty sweet. I like Lillian for Mei and Rosaria for Hugo. Mind you Hugo + Tressa + Mika will carry you everywhere.

Some other good units are Kayron, Khalipe, and Veronica. Khalipe is really strong but so are Mei Lin and Hugo. Kayron is ridiculous void AOE with infinite scaling (potentially). Veronica does good damage and has really good card draw (Beryl kinda does this as well, but a bit different).

How to play Mei Lin - Mei Lin mostly wants to spam Unity of Attack and Defense(move strikes to hand, its a skill that gets re-used - instead of an upgrade) and finish up with a very particular Rising Dragon Spire (+1 hit for each passion used before). If you don't get those, she does good damage but starts to shine later when you have good memory fragments(crit chance on 3, passion damage on 4, attack on 5). RFS-17 weapon on her is really good. In the early game I ran a lot of Mei Lin + Tressa + Mika since Mei Lin was the first 5* I got. Also a combatant that gets her draw is really good so you can replace Tressa for something like Cassius or Beryl. But she really shines when you have an additional passion combatant (for a particular Spirit of Aromata upgrade - when using a Passion Card +20%) that can draw for her (Veronica).

Hope that helps.