There are ten required fields that you can choose to hide or show when you're adding a new relationship/partner for an already existing contact:
Title
First Name
Surname
Either First or Surname
Address
Mobile
Home Phone
Email
Either Mobile, Home Phone or Email
None
To update, head to your office settings > Preferences > Office Preferences > Contacts > Toggle On/Off for 'required fields when creating a contact's relationship/partner'.