Additional needed documents for UK trip if someone will shoulder all expenses

What are the documents I need to submit aside from bank statements?

Aside from photos and your passport, there is no 'set' list of documents; each applicant decides what to include. To do that, you need to examine the Supporting Documents Guide and determine which documents will reveal your personal circumstances most favourably. Your bank statements will be important despite being co-sponsored because they will show ties to your home country.

You can also benefit by reading the rules and deciding which documents improve your chances of qualifying. Pay close attention to V 4.2.

Does the sponsor need to fill-out the Sponsorship Undertaking Form?

No. The form you linked to is out-of-date anyway and the rules about sponsorship have changed since that form was created. So the only thing you can accomplish with that form is confirm to them that you did not study the guidance (that's a bad thing).

my relative is a naturalized citizen in the UK. Is proof of naturalization needed?

No. Being a citizen is not a requirement for sponsoring someone and proof of your relative's immigration status is not needed. However it's 'best practices' for sponsors to provide a photocopy of the portrait page of their passport.

They will be much more interested in the your relative's motivation for sponsoring you than they will in their immigration status. They will also be interested in their sponsorship history.

You didn't ask what the sponsor should include in order to establish their capacity for sponsorship, but briefly they should also adhere to the Supporting Documents Guide, including the landlord's permission if the sponsor is not a home owner.