Nihil as the embodiment of the tendency to escapism
There is nothing in the world more helpless and irresponsible and depraved than a man in the depths of an ether binge. — Hunter S. Thompson, Fear and Loathing in Las Vegas
I really dislike this character, but I will try to be impartial as much as possible. So, Nihil is an illustration of a severe escape from reality, a desire to bring back the past, to stop time. In the era of the Third, he brought back the Middle Ages as Papa Zero. In the Cardinal era, he was slipping his old records onto store shelves (so people would know about them? Or rather to create the illusion that they were for sale again?) and regretted not being allowed back on stage as a frontman. Hence his envy of Copia, his constant belittling of him, emphasizing that he hadn't achieved some results and comparing him to himself. Nihil felt that this place legitimately belonged to him, with no regard at all to his health capabilities. Eventually, he got three of his old songs performed in the era of his successor.
Escape into illusion is the main way Nihil interacts with reality. Immersing himself in his enchanting fantasies where everyone is young and beautiful again, he overlooked a fateful decision and allowed his sons to be killed. The illusions are like a membrane through which he perceives the world around him. Sister got him to appoint Cardinal as frontman because from the depths of his erotic fantasy he answered ‘yes’ to her and that became an answer to her question. In fact, he did not make any decision.
Maybe inhaling oxygen from the life support system is having that effect on his brain. Sometimes I think it's not just oxygen, but something else mixed in. He takes a breath from the tank when he sees Sister wrapped in bandages and confined to a hospital bed - and the semi-darkness of the room immediately turns to sunlight, everyone becomes beautiful and happy, and Sister flirts with him instead of wheezing. Reality becomes bearable. We also hear the sound of breathing into an oxygen mask (similar to Darth Vader's breathing) in the background of the scene in episode 7 when Nihil sees the 'ghosts' of young Sister and himself. Perhaps all that is happening is his hallucination caused by inhaling ether.
Nihil is so incapable of accepting his current moment of existence that he always keeps his oxygen mask at the ready. He probably sees Sister as the strong, active person who will contact reality in his place while he enjoys his fantasies. We know what Sister is doing behind his back: killing his children and promoting her son, giving him as much patronage and privilege as she can.
Eventually, in the RHRN movie, Nihil seems to finally become able to make contact with reality: he is already a ghost and no longer uses a breathing support system. Sister makes him remember his paternal duties, and he gives parental guidance to Copia. Along the way, he realizes his place in life, in his family and at work. He is finally here and now, not in his fantasy world. It took him dying to do it.
Well, better late than never.