No — not permanently and instantly without paying or playing.
If you are looking to unlock all characters (Legends) in Brawlhalla
The most straightforward way to unlock every current and future character is to buy the "All Legends Pack" or the "Founder's Pack" through the in-game store or Steam.
To unlock all characters (known as Legends) in Brawlhalla on PC, the most effective method is purchasing the All Legends Pack