Since his father, Kusakabe, had died as crown prince before enthronement, Karu should have normally been addressed "王" (referring to male imperial family members) rather than "皇子" (referring to emperor's legitimate sons or emperor's blood brothers), but it may have been due to his grandmother Empress Jito's guardianship that he had supposedly been treated as the latter before he truly became the crown prince.
父草壁は皇太子位のまま亡くなり即位していないため、本来であれば「皇子」ではなく「王」の呼称が用いられるはずだが、祖母である持統天皇の後見もあってか、立太子以前から皇子の扱いを受けていたと思われる。