Since Lucrecia is his mother, I'd imagine his last name is Crescent and Lucrecia and Hojo was never married.
But he is a Shinra secret so he may not really have a last name plus he consider JENOVA to be his mother. In conclusion, no last name for Sephi.
BTW, I like Chicken.
Not every culture has the whole first & last name rule, maybe they do shit different in FF?
Not in FFVII. (Cloud Strife, Tifa Lockhart, Cid Highwind, etc..)
It's often said his last name is Crescent, but that's a purely fan-given term.
I prefer to think of him without a last name.
He has no last name. Sephiroth is a designation he was given by Shinra and hypothetically could be said to not even be his real name. Likewise surnames are given by profession, notable qualities, who you are the son/daughter of, and/or passed from parent to child and this is usually only done when there is a need to make the distinction. Given that Sephiroth is a designation and likely isn't a common name...at least till only a few years ago in their time, he has no last name or need of one and likely didn't have his name pass from parent to child on "official" forms.