The Hot Week we are having, and the next one, give 10-Year Tokens, which can be used to buy things from the Artifact Keeper NPC in Event Hall.
One of these things is a character slot, for 20 Tokens.
The purchase limit is one per character.
Which means you can claim the Hot Week boxes on different characters, and get a character slot every other day.
There was discussion in the official forums about whether this is an exploit, and Artasi responded