When contributors click to contribute they have several options to choose from.
They can contribute as regular user with their information visible on the backer list, they can contribute either fully anonymous (where all of their information is hidden from the backer area) or partially anonymous (where some of the their information is hidden from the backer area).
In addition to these options they can also choose to contribute as special guest user. When contribution is made as guest user the user account is not created (because user has not supplied password) so contribution gets allocated to special portal user called “Guest User”. All contributions going through “Guest” option get allocated to this single and special “Guest” account.
Note that only difference between “Guest” contribution and “Regular Contribution” is that password field is removed so that account is not created. If you intend to accept regular contributions and want users to be created then please disable “Guest” mode and allow people to contribute in a regular way where they can select “Anonymous” contribution option if they do not want their information to be shown or shared.
Any of these modes can be enabled or disabled. For example you can disable guest contributions and allow only regular contributions. This is all controlled from the Administration Panel for your website portal.
You can also change name and email of this special “Guest User” by navigating to the Portal Administration -> Users interface. There simply locate the Guest User and edit that user. Make sure to never remove this special user.