Their presence was necessary, as neither the Devils nor the Angels would let go of their own Werdlem Treasures. So, Soul Gem had to invite them so they would bring the Treasures over. As for the Werdlem Hexahedron, Soul Gem already had it himself.
At first, the Archangel and the Elder Devil were skeptical. From what Soul Gem told them, Rean and Roan were actually not dead. Also, Kentucky and Celis, who simply disappeared all those years ago, were actually with the twins.
The loss of Rean and Roan back then weighed quite heavily. Fortunately, their 'sacrifice' was what helped the Lisan Universe to finally win the war against Garisan. Now, Soul Gem needed the power of their Werdlem Treasures to be able to bring them back.
