Seth

Seth is one of the Castor brothers, the military clone line that mirrors Sarah's world from the other side. Introduced as part of that tight, regimented brotherhood, he carries the clipped discipline and unsettling family resemblance the Castor men share, another face in a project built on control and conformity.

Ari Millen's real feat on Orphan Black is making each Castor brother distinct within a single body, and Seth benefits from that specificity — a small variation in posture or temperament that reminds you these are people, not just plot devices wearing the same face.