Could you recount the nine-year-old story behind the portrait of Sai mentioned in the text?

The story behind the portrait is one of a long-held secret and a promise fulfilled after nearly a decade. According to Chapter 41, the narrator and a priest named Ismu made a private, firm decision nine years earlier that a specific portrait of Sai should be offered by the narrator to its intended recipient. On that day, the narrator presented the image but had to leave quickly and couldn't share the backstory. He intended to tell the story later, but a full nine years passed before an unexpected meeting provided the opportunity to finally explain the history and the original determination behind the gift.