You should reach the Six Paths Sennin once so that you can receive the corresponding rewards based on your level each season.

Guys, before complaining, please read the arena rules. Make it a habit to read and understand each and every rule in a game. The arena said, "Reach six path sennin once". I personally also dont like how this system works, but it is not made just for p2w imo. It is made so that you spend more time in the game, which I dont have. But I agree that basic f2p ninjas have almost 0%chance on winning in arena. Maybe Im one of the lucky f2p guys who got Shisui because he is very good in arena and helped me reach 6 paths in no time. Just play wise, save your coupons if you are f2p like me, and spend it on events where you can get better ninjas for arena.