Reason behind the knocking on the door by Tengo’s father?
Tengo’s father was good only at collecting fee. When he was in coma someone keep banging on Aomame’s door and later on the door of Ushikawa also to collect fee. It was Tengo’s father. His father was sad and lost interest in life. In Tengo’s words
“I think you lost all interest in this world. You were disappointed and discouraged
and lost interest in everything. So you abandoned your physical body. You went to
a world apart and you are living a different kind of life there. In a world that is inside you.”
Tengo was not his real son still he kept Tengo with him his whole life because he loved his wife and hoped she would come back one day. So he lived his life on this only hope. But she never came back. So this banging on the door demanding payment by this ghost fee collector is embodiment of life’s lost souls and unpaid debts.