open, begin, commence, explain
启 centers on opening something up, whether physically opening a door or starting an event, and by extension explaining or revealing what was closed or hidden.
启 combines 户 (door) and 口 (mouth/opening), suggesting the idea of opening a door or beginning to speak. The exact historical development is uncertain, but the component combination fits the core meaning.
A door (户) with a mouth (口) inside: imagine opening a door to speak or to start something new.
For ケイ, picture opening a door with a key (ケイ): the key turns, and the door opens to begin something.