Who is responsible for the TM30 registration if I'm not on the rental agreement in Thailand?

The landlord is correct that he doesn't have to file the TM30 if you are not on the rental agreement. However, it's crucial for someone, likely your girlfriend as the possessor of the property, to register you to avoid potential fines. The TM30 should be submitted within 48 hours of arrival, and failure to do so could result in an 800 Baht fine. Ensure that the registration is done promptly to avoid any issues with your stay.