In Briar Rose, Josef Potocki was the man who saved the life of Rebecca's grandmother. He gave her the kiss of life after she was recovered from a pile of dead bodies (which were the bodies of recently gassed Jews/Gays/Gypsies). Not only did he save her life, he also became her friend. He loved her, in a sense (although not in the intimate way [Josef was homosexual]). Even decades afterwards, he still remembered the red haired girl named Princess.
I would not consider Josef Potocki a minor character. In my opinion, he is nearly as important as Stan. He is most certainly a major character. He narrates more than one third of the book and is the one who saved Rebecca's grandmother. I believe this constitutes his as plenty more then a side character.