Klaus Barry Heterodyne was born in "1_72" to Lucrezia Mongfish Heterodyne and Bill Heterodyne. He was named after, first, his father's best friend Baron Klaus Wulfenbach, who had disappeared two years and three months before the child's birth; and second, his uncle, Barry Heterodyne.
Neither the fact of his death, nor Agatha's subsequent birth, was generally known in Mechanicsburg. Both Klaus Barry Heterodyne and Lucrezia Mongfish would most likely have been missing and presumed dead after the attack.
The fact of Lucrezia's pregnancy at the time of the attack was likely not generally known; in addition, the Heterodynes rarely father girls. Thus any speculation about a lost Heterodyne heir would center on the survival of Klaus Barry Heterodyne; if the child with whom Lucrezia was pregnant at the time were considered in any speculation, that child would have been assumed to be male as well.
The absence of an announcement of Klaus Barry Heterodyne's death could simply have been the result of the absence of the Heterodyne Boys after they dealt with the immediate aftermath of the attack, and Carson von Mekkhan going into hiding afterwards, or it could have been the result of a general policy of limiting the amount of information available to outsiders, or done specifically to protect Carson, any Heterodyne heir to which Lucrezia might give birth, or both.
Von Pinn served as Klaus Barry's "nursemaid" and went mad at his death. We later that she existed for about 200 years before Klaus Barry's birth, having been the Muse Otilia created by Van Rijn for the Storm King and later transferred by Lucrezia into a construct body that could not resist Lucrezia's commands.
His younger sister is Agatha Heterodyne, although she did not find this out until she was eighteen — just as many who knew Klaus Barry were in turn unaware of her.