
Sometimes I wonder if we're reading a sequel of Fairy Tail. This character development demand is unrealistic to me.
I don't see any reason for her to 'mature'. She is not immature. She has her quirks, and is cute. Nothing wrong with that.
Laxus is a FT member and grandson of Makarov. Ultear and Jellal were the first characters to appear in the manga. Comparing those 3 to Brandish is apples to mangoes.
Brandish has received enough character development already. Just look, she's able to smile about competing for a key that was at one stage a source of sorrow. For a FT character - and a minor, essentially irrelevant one at that - that's par at least.
Character development is mostly discrete in FT, and usually happens the one time. Laxus and Jellal got their one big change, and ever since have been humming the same tune. Not particularly amazing, but it's enough to say 'yea, good on you Laxus'. Brandish has less to offer the story than those two. To me, she's basically here to give Lucy the push along - not for Brandish to become the star (no pun intended).
Granted, we'll get more panel time from Brandish, and maybe there we'll get this 'character development'. Maybe she'll be so amazed by FT that she'll go back to Alvarez and reunite the remaining 12. Yaaay... character development, yaay.