While "Summoned" are pretty much just the hero thing in the Saga Empire, there's a ton of reincarnated characters.
My favorite are the underground monsters.
Spoiler:
Basically a parody of Kaibutsu-kun, we got trio of monsters, vampire, golem, mummy, and a goblin princess living and ruling kingdoms underground. They are all reincarnated Japanese from centuries ago.
Heck one of the monsters, I think it was the mummy, even created industrial revolution when he was still human, and the gods drove him to flee. He actually THREATENED the gods that if they don't leave him alone, he'll blow up the world with nuclear weapons.
So they let him be, as long as he doesn't return to the surface world.
So yeah, having a previous life is not super rare in this world, although most "second-lifers" became legendary beings.
Heck, the combined powers of the monster quartet could possibly even give Satou a run for the money.
Good thing they are total bros with Satou.
Satou is a special case though. He's neither reincarnated or summoned.
Reincarnated starts off as a baby, and gets interviewed by gods before birth.
A summoned hero comes to the world exactly as themselves. Satou is neither, he found himself alone in a younger body.