Per wiki policy, Spoilers Off applies here and all spoilers are unmarked. You Have Been Warned.
Sakamoto is a deity of some sort
He, in fact, is the literal Deus ex Machina. But gods need knowledge of human world to do their job properly. So Sakamoto being himself, decided to start like all humans do - from school. Sakamoto is a demon.
Of the same variety as Sebastian and Claude from Black Butler. Since he's not in a contract with anyone, he seems to be attending a human high school for fun, or perhaps he's a young demon undergoing training. Sakamoto isn't a demon.
He isn't a demon. He's a young kitsune who goes to human school for some reason. Maybe foxes need a good education too? Sakamoto is a half-demon.
And half-Umbra Witch on his mother's side.Sakamoto is dying from a chronic illness.
Let's break it down:- Sakamoto often goes to the nurse's office, and going by how his only regret is not knowing how many holes are on the ceiling gives away that he's there that often.
- In the Manga, Sakamoto is seen lying like a corpse inside a locker as if it were a coffin.
- The song he sings inside the bar is Der Erlkönig by Johann Wolfgang von Goethe where the song is about a boy killed by a supernatural being.
- Actually the song is about a feverish boy dying of illness in his father's arm on the way to the doctor, while hallucinating that a fairy king thinks he is beautiful and graceful and perfect and the fairy king's daughters all love him. It's even worse, implying that Sakamoto is hallucinating that he's perfect.
- Sakamoto tells Acchan how he has never seen snow in his life before, which would make sense if he was in the hospital for his whole life.
- He's a Chaste Hero because he knows his critical condition will cause a lot of distress for his partner, so he prefers to turn them down.
- Finding life in Mars is an almost impossible goal to achieve and NASA would likely not hire a High school freshman in their project.
- His whole graduation speech is about charging straight ahead when you have a choice forced upon you, and how you can believe in your friends if you no longer believe in yourself.
- Speaking of the speech, the school decided to let a freshman do the speech instead of someone who's graduating.
- It's actually a tradition to have a non-graduating student give a speech during graduation in Japan. That being said, it's usually a junior not a freshman so your point still stands.
- He tells Fukase that he will "Jump into the Night sea and find a new continent" which is a metaphor for death.
With all that, he's likely the person we knew because he wants to leave an impact on people before he passes away, but he doesn't want people to get emotionally attached to him before they realize his illness, hence why his backstory and family are unknown.
Sakamoto is an extraterrestrial.
The OVA shows him standing on top of a tower somewhere in America. So he wasn't lying about going to America, but the real reason he went there was because he had to stand on top of that one tower in order to make contact with his home planet. Sakamoto collected his data about Earth Japanese High School and went home. Sakamoto is related to Mary Poppins.
He's practically perfect in every way and at one point even uses a parasol to fly. How much more proof do you want? Sakamoto is related to the Joestar family.
And his Stand is what allows him to do everything with style. Sakamoto's first name is also Sakamoto.
He did say Sakamoto was his full name in chapter 4, after all. Sakamoto is a Super-Soldier at the wrong place.
