do you have other healer? if none. then i vote for 6* angelica first. 

Luna

I’d say go for the healer first. So Angelica 

Since you are still starting, prioritize in building your wyvern 11 team to be. (Gold farm and universally useful gear)

Angelica, krau, luna are your options, i'd recommend to 6* first luna then Krau for tank, angelica can remain as lvl 50, as she can do most of all the content even at that level.

Of course you can simply ignore any of the recommendations and play the way you want it to.

Depending on what you want to do next. Ideally you want a hero with spammable AOE who can help you grind, but that does not apply here.

As suggested above Luna is a super solid choice and I would totally agree, she will help you in quite a few ways, even low-mid level PVP.

After Luna it'd be Angelica for continuing advancing your wyvern team, or Krau/Carmin/Basar for PVP.

For now Luna definitely won't go wrong. Also, no mistake in this game is permanent, EVERYTHING can be corrected with time, even limited banners that you missed. Just do what you feel is fun and enjoy the game.

I'd vote for Ravi because she can solo most adventure stages and speed up your levelling process