For those who purchased and preloaded the game before that will have to manually download the character with a separate download code from the Nintendo eShop. The codes should have been sent via email today (July 18) to the email address associated to the Nintendo account. Here's a sample of the sender and subject for such emails:
Thanks Bui from the Discord for letting us use this screenshot
Inside the email should be two codes: The 16-digit keep the Reference Code (For Inquiries) (管理番号(お問い合わせ用))
If you never receive the email where you should have, you should contact BNEI Support with the contacts in the notice, quoting your 21-digit Support Number (サポート用の番号), accessed from your Switch by My Page > User Settings.
Notice on NS1 main site