[advance]

Your personal tastes are not translation. Sorry.

Translation is about narrowing the amount of interchangeable words to find the appropriate equivalent. This is why you can't use something so informal as an "equivalent" to a term that carries some respect. No "teach" or "prof" or "boi" could be used to show respect to an upperclassman. "Sempai" is a term used for upperclassman in schools, coworkers with more experience, etc. It also isn't too formal because there have been friends who are more experienced in some places who are called by such a term. "Sir" is not a direct equivalent. "Upperclassman" seems to be the closest one, but people aren't accustomed to hearing it as a title.

"Masato" never is a translation. It is a transliteration at most.