That aside, I have a theory that Sun-fei's master adopted him or was a blood relative of his that died early on in his life.
There's two factors to support that theory:
1. Sun-fei constantly keeps up a level-headed image of himself and mentions "maturity" & "growing-up" frequently in the early chapters. This is usually shown in people who were forced to "be an adult" as kids due to circumstances where an adult figure (parent, guardian, etc.) is lacking in their life, and they have other people to take care of but sadly no one else to fit the bill.
2. The position Sun-fei holds at Red Chanpuru is a major one, deduced by the fact everyone in the island knows him. "Why give such a considerable position to a kid who hasn't even fully graduated?". Inheritance/Succession is the answer. The person he likely gained from was his master, who is long deceased.
(extra note: I went back to reread the early chapters, and I have a feeling that Sun-fei gets envious of people who are able to willingly open up about themselves. Sometimes he wears a look of 'longing' that gives the vibe that he struggled with similar situations but never has it in him to actually tell anyone about it.)
LMAO the amount of comments I've seen bashing Sun-fei re-surged again, y'all are so predictable Trust they'll end up talking it out, it's how Nii Satoru operates