People keep asking what Jacob and Nini looked like as children.
That is also Jacob's true form, though with more adult proportions now that he's older. He can only add to this form, but not take from it.
He puts on a more demonic look with horns and what not because the asuras that raised him taught him it's best to be feared and to intimidate other demons. He's still very very strong through, so people are right to fear him. The demonic look is really just to avoid being challenged constantly and because it fits his personality better. Call it a fashion choice if you like.