You can specify that system fields and user-defined fields are mandatory. System fields are set as mandatory in Administrator. User-defined fields are set as mandatory when creating the user-defined fields in Administrator or in the Set Up User-Defined Fields dialog box.
Mandatory fields are denoted with an asterisk displayed after the field type or with a red border.
When you create or modify entries, Maximizer checks that all mandatory fields have values. If any mandatory fields are empty, you are prompted to enter a value for the field. If you are working in the Details pane, any user-defined fields that are empty are added to the current Key Fields list.
You may not be able to remove last_name from mandatory check. Last name is one of the main attribute that we use on multiple locations (Web Forms, Leads, Contact List, Accounts, Client Service, duplicate checking, import) and has reference every where in the code(back end or front end).