It never says, all we know is that Arika's mom had 'single mom' status, so we presume he's either dead or simply left. Considering everything that has to happen to convince an Otome to give up her position ie: have sex, we can assume it's not the latter, but the how and why of the former seems equally unlikely.
Of course I could be wrong, could just e it was never deemed important to show him, who knows
14-May-2006