If you created a pixiv account using an Apple Account (Apple ID), follow these steps to log in.
■ How to log in
Go to the pixiv login page. Click "Log in" and select the Apple logo under "Log in with an existing account". Enter your Apple Account login details.
If you can't remember your Apple Account password, reset it using the following link.
■ Reset your Apple Account password
https://iforgot.apple.com/password/verify/appleid
If you can't log in with your Apple account...
First, try logging in with the e-mail address where you receive e-mails from pixiv (e.g. updates, announcements, and so on).
If you can't log in with any known e-mail addresses, check if your pixiv account is registered with an "@privaterelay.appleid.com" e-mail address.
You can do so as follows.
■ How to check your registered e-mail address
1. On your Apple device (iPhone, etc.), go to Settings > Apple Account (your account name) > Sign in with Apple > pixiv
2. Find the address ending in "@privaterelay.appleid.com" under "pixiv" and use it to reset your pixiv password.
■ Reset your pixiv account password
https://accounts.pixiv.net/password/reset
After resetting your password, try logging in with your "@privaterelay.appleid.com" e-mail address and new password.
If you created a pixiv account using an Apple Account (Apple ID), your registered e-mail address might not be the one linked to your Apple Account. If you enabled "Hide my e-mail" when signing up with Apple, Apple automatically generated a random "@privaterelay.appleid.com" address for your pixiv account.
Any e-mails sent to this address are automatically forwarded to the e-mail address linked to your Apple Account.
To learn more about privacy settings, please check Apple's Help pages.
https://support.apple.com/105078