Superhuman Stamina. I've played games where the protagonist can fuck with many girls, makes them cum, causes them many orgasms with the capacity of keep on going without taking any important break in the course of this events. They can keep their erections hard even after cuming more than two times, again, without any break or physical sequels due to the overexertion. Characters with the greater stamina are those whom can fuck with supernatural entities such as Succubus, Monster Girls, or Demon Girls, because they are supposed to be creatures with a power beyond any human in the planet, yet some protagonists can fuck them 'till the point of make feel them in heaven.
This power is probably ignored by many 'cause the protagonist who can have 'em all is a cliche in basically any harem history, so sometimes is underestimated the energy, and even the will needed to do something like that.