Schengen Visa for Boyfriend Under "Visit Family/Friends" Category

I’m an Indian planning to visit my sister in Germany this year. She’s a permanent resident and will provide an invitation letter and a formal obligation letter (Verpflichtungserklärung) for my Schengen visa application under the "visit family/friends" category.

My boyfriend is accompanying me on this trip. Can he also apply under the same category using my sister’s invitation letter? Is it mandatory for my sister to provide a formal obligation letter for him, or can he show his own financial documents (bank statements, salary slips, etc.) to prove financial independence?

If anyone has experience or advice on this, I’d really appreciate your help!

Thanks in advance!