Can I use Seeker if either player has no Benched Pokemon, or do both players need to have at least one Pokemon on the Bench?
If there's a Pokemon on either player's Bench, Seeker can be used.
Source: Triumphant FAQ (2010-11-04), PUI Rules Team (2010-11-04)

Relevant cards

Note: All applicable cards may not be listed below.
HGSS4_88