I just wonder if I unlock any chars like Samurai etc. and when ladder gets reset, do I have to unlock them again?

I play since 1 week so I don't know. Didn't find anything about it here. Thx for help!

all unlocked characters are lost

and all characters are rerolled

thx

I just figured out that a ladder season would be very short if it were not like you wrote.

Very true.

You keep all your items, proficiencies, statistics, etc. The only real change is everyone's characters get rerolled back to the start, ready to do it all over again.