Upgrading your Brawler level increases Brawler's state. Once your Brawler reaches level 9, Star Power will be unlocked, so that you will get the additional ability. To upgrade Brawler's level, you need two material; Power Points and Coins.
Brawlers Upgrade Required List
To get a Brawler to level 10, you must obtain its Star Power. To find a Star Power, your respective Brawler must be at level 9. Star Powers are found in Brawl Boxes, or they can be bought for 2000 Coins in the shop when they appear.