The special relationship and friendship that existed between Rudeus Greyrat and Nanahoshi Shizuka could have occurred when he was at Ranoa Magic Academy and he met Rudeus Greyrat.
Where he was always with Rudeus since when he had to sleep for several years, and was only awakened for 12 days each year so that he could survive the several hundred years that Mushoku Tensei’s story was running.
He gave advice to Rudeus and also received some advice from Rudeus, which was to think positively about the future until death awaits him or Nanahoshi Shizuka in the future.
The disease he suffers from is called “Drain Syndrome”. This makes him the unluckiest person in the world of Mushoku Tensei, because his breath can be very deadly to other people.
And he also couldn’t have children in that world, so now he will decide to commit suicide if Rudeus Greyrat doesn’t calm him down within a few moments. When Rudeus Greyrat reached the age of 75, and Nanahoshi Shizuka, who heard the news that her friend in the same fate had died, was devastated and couldn’t hear anything when Roxy and Sylphiette won her.
During the storyline after the death of his best friend and his fate, he is with his friends’ great-grandchildren to defeat Hitogami from his world and save the world with Orsted. Several decades after the death of his friend, Nanahoshi often visited the grave of his friend who was peaceful in his grave.