Can I get pre-registration rewards?

When you start a game of Arknights and finish the tutorial, check your mail to see the list of rewards that you have. As of the game's release date (January 16, 2020), all players get pre-registration rewards as long as a new account is created.

If you are doing a reroll as well, you'll still get these rewards even on guest accounts. Take note though, that this may be subject to change in the future.