Also, side note: Did anyone notice how Steven has a last name but not our boi? That's downright disrespectful. Another side note: What is Wallace's last name anyway? If you know can you tell me?
A majority of Pokemon characters don't have established surnames. The only reason we know Steven's is because we meet his father, and I don't even think they established his FIRST name in the original games (It's Joseph, btw)
Steven is, to my knowledge, the only non-Professor (or character _related_ to a professor, like Blue) in Pokemon to have a surname, the only other exceptions I can think of would be Ghetsis, N, and assumedly Anthea and Concordia, all of whom have the surname Gropius, along with those families like the Winstrates from Gen 3, whose individual members all have given names and surnames.