He has the appearance of a somewhat eldery, balding man and generally shows a very pleasant and friendly demeanor.
He is a very calculating man.
He is one of the 3 pillars of the Radiant Church alongside Osseno and Lehman and, as such is aware of the dark secrets of the Radiant Church. He is actively engaged in sacrificing pure souls to the Radiant Sovereign (including the soul of Linley's mother) and procuring strong bodies which have no battle qi to be used to house decending angels (the original purpose the church had for the Barker brothers before Linley rescued them).
Heidens dies at the hands of Linley when the sacred isle is invaded (and destroyed).
He is a peak level saint who has at least one divine artifact (the divine scriptures). He is a grand magus saint using Oracular magic (one of the 3 high magics alongside Necromancy and Life magic) and is, as such, an incredibly powerful figure in the Yulan continent, only lagging behind the prime saints and deities when it comes to power.
He also has a trump card where he can, at the cost of all his spiritual power and some damage to his soul, summon the impression of a powerful radiant deity. This impression overwhelmed Linley (at the time a prime saint), Delia (at the time an incomplete demigod) and Bebe (at the time a peak level magical beast) and was only destroyed by a mysterious force when it was about to kill Bebe (presumably this force was Lord Beirut). This, along with his other powers, implies Heidens could have killed any of the saint level beings on the Yulan continent as long as he had sufficient time to activate the summoning spell, which would be incredibly unlikely in a 1 on 1 fight.