Hello Xabier Torrado Rey,
Thanks for reaching out here in the Microsoft Answers Community.
The "WSIAccount" user is a system account that are used for Temporary Access Pass, which is used for Web Sign-in.
Please do not remove such system accounts, as certain system functions may run as, or depend on such accounts.
For further details regarding Web sign-in, please refer to this Microsoft Learn article.
Thanks for your patience and understanding.
Best Regards,
Sheng G. - MSFT | Microsoft Community Support Specialist