For boys and men, typically no underwear is worn beneath the garments, however it is acceptable to wear briefs or similar. This is helpful to children in the event they make and error and the Hakama comes loose.
Additionally for Girls and Women it is common to wear some sort of sports under garment both to and bottom. However be sure to keep them black where possible or indigo blue. And try to stick with designs that will not be visible from beneath the Keiko Gi and or Hakama.