I DO ACTUALLY but uh preliminary warning of heavy topics and suicide mentions because, spoiler alert, i don't think they're alive.
my hc for them is that they were a young couple (late 20s or so) who were very deeply in love and were (especially on the mom's part) very excited to have a son together. for appearance, i hc his dad as a very plain (not unattractive, but just a bit boring. you'd forget about him in a lineup of people) man, while his mom i hc as an extremely pretty woman who basically just looks like him as an adult but with a smaller frame, rounder eyes, and long hair. so yea they were a very loving couple who were ecstatic when they realized they were having a child, spent all 9 months buying new things for the baby and preparing a comfortable family home. unfortunately tho, after satoru was born, within the first year of his life it became apparent that he wasn't a healthy baby. as an upper middle class family, the parents did everything they could to keep satoru's life comfortable and to fund tests and research into what could be wrong, but obviously it was very mentally taxing for them after years of little progress. at some point, they find out that satoru's condition was a result of a recessive gene carrying over from the mom, which devastates her; she begins to struggle with mental health, still doing her best to keep her health in check (therapy, medication. etc) with her husband's full support, but all of the self-care taken doesn't stop her from spiraling into a very depressive state that centers around her guilt in being the cause of satoru's suffering, and she eventually commits suicide unexpectedly while the father wasn't home. the father was already struggling (a son weak in the hospital and a wife who he loved dearly struggling every day), but his wife's suicide completely destroyed him, and within a few months of his wife's funeral he gets into a deadly traffic accident in which he was intoxicated and it was unclear whether it was intentional or accidental on his part. so satoru's an orphan fairly young, around age 5 or so. the last memory he has of his father is of him sobbing at his bedside after the funeral, and he struggles to remember his mother's face as anything that wasn't clearly sorrowful. as a child, he was told that both of his parents died in accidents, but later in life (as a teenager, after he was released from the hospital and began sorting through his parents' things) he read his mom's death certificate and found out it was a lie, which he understood was just to not have to explain the idea of suicide to a child, but did "confirm" suspicions in his head that his parents would have been happier if he'd never been born. he can only remember what both of them looked like in happiness by one family picture of them together when he was an infant.

after release from the hospital, satoru was taken care of by non-immediate relatives (mostly his grandmother until her passing) and friends of the family, who all promised him that his parents loved him very much and would be proud of him, but satoru could never shake the idea that his parents would still be alive, and the people who loved them (and him, by proxy of loving them) would be happier if he'd never been born at all -- exacerbating the thought that his friend's death was also his fault.

so like.. counting his grandmother, satoru experiences 4 heavily significant deaths before like, age 16. yada yada tragic widow who is followed by death imagery, he is currently trying to separate himself from the thought that he was a burden to all of them or somehow caused their deaths, but it's one of the deepest anxieties that scratches at his brain.