In the case of Valeria/Kasumi, they already have allocated stars from the previous Suikoden. As characters never change their stars, Tomo provides a filler for the other one. So if you recruit Kasumi, Tomo becomes the Tensyo star in place of Valeria. If Valeria is recruited, Tomo fills Kasumi's space.
The other three are monsters - you only choose two of them to recruit. Whichever two you choose will occupy star 12, Tenman, and star 25, Tentai.This
explains it better.
The only thing Suikoden lacks...is dinosaurs.