Master Fu had been the guardian of the Miracle Box since the end of the other guardians. When a life-or-death situation struck at the end of the third season, he passed on the guardianship to someone else. Master Fu had lost his memories when he passed on the guardianship of the Miracle Box to Marinette Dupain-cheng or, more famously known as Ladybug. I have attached the video here if someone hasn’t seen the last episode of the last season:
Master Fu now remembers nothing about his past or present that is related to Miraculous. Unfortunately, Marinette is having a bit of trouble handling everything. From Studies and Adrien and Luka to her Miraculous and the Miracle box, she still wishes for Master Fu to come back.
What I think that if Master Fu’s memories come back he would act as if it didn’t so that Marinette can learn to handle the problems on her own and would just come to visit her and secretly the kwamis to just check on her and see if she is fine. He would also judge her choices of miraculous holders like giving (SPOILER ALERT) Juleka the Tiger Miraculous or giving Rose the pig miraculous. He would only interfere when absolutely necessary and that also indirectly. He will try to keep his facade of being an old person without any knowledge of Miraculouses. Though I think that he would also try to interact with Adrien indirectly and try to make him feel not “lonely” and “without his lady”.
He would also love to talk to The Great Guardian of the Miraculouses, who makes an appearance in the episode, “Furious Fu”.
So in the end, I think he will be a type of over looker to the miraculous holders and would judge Marinette to check if he had made the right decision.