ai ries ▁of ▁the ▁series ▁live ▁under ground . ▁In ▁this ▁way , ▁the ▁name ▁Kob oi ▁su its ▁the ▁gold - d igger ▁Op al ▁Kob oi . ▁It ▁is ▁also ▁known ▁that ▁Kob oi ▁bank rupted ▁her ▁father ' s ▁business ▁after ▁he ▁tried ▁to ▁dis su ade ▁her ▁from ▁studying ▁engineering ▁( as ▁he ▁expected ▁his ▁daughter ▁to ▁follow ▁the ▁normal ▁path ▁in ▁life ▁for ▁female ▁pix ies : ▁namely , ▁marry ing ▁a ▁suitable ▁husband ) ▁and ▁that ▁she ▁has ▁a ▁long , ▁bitter ▁rival ry ▁with ▁Fo aly ▁since ▁their ▁days ▁at ▁university , ▁and ▁that ▁one ▁of ▁Op al ' s ▁main ▁goals ▁is ▁to ▁prove ▁she ▁is ▁intellect ually ▁superior ▁to ▁the ▁cent aur . ▁The ▁only ▁reason ▁to ▁this ▁hat red ▁between ▁the ▁two ▁mag ical ▁cre atures ▁is ▁that ▁Fo aly ▁won ▁their ▁University ' s ▁science ▁fair ▁with ▁his ▁ir is - cam , ▁which ▁Op al ▁found ▁stupid ▁and ▁to ▁which ▁she ▁thought ▁that ▁her ▁Kob oi ▁wings ▁were ▁far ▁superior . ▁She ▁then ▁acc uses ▁the ▁cent aur ▁of ▁winning ▁only ▁because ▁he ▁was ▁a ▁male . ▁ ▁Sub sequ ent ▁appearances ▁Op al ▁Kob oi ▁returns ▁as ▁the ▁tit ular ▁ant agon ist ▁in ▁Ar tem is ▁F ow l ▁and ▁the ▁Op al ▁De ception , ▁which ▁was ▁ ▁published ▁in ▁April ▁ 2 0 0 5 ▁as ▁the ▁fourth ▁book ▁of ▁the ▁fict ional ▁series , ▁following ▁her ▁absence ▁in ▁Ar tem is ▁F ow l ▁and ▁the