Souyouju = Evil Twin Tree, the tree Chibiko and Hii-chan grew on. For details, see Welcome, Newlyweds 2

Seikoseki = Star Fox Stone, the magic stone Hiei got to try to make Kurama obey him. Chibiko and Hii-chan each swallowed half of it, and transformed from plants to real kids. For details, see Welcome, Newlyweds 3