The sixth season episode "Two of Hearts" is the only episode to not feature any regular cast members (including the titular Highlander, MacLeod).
The character Katherine was the fifth female immortal introduced as a possible prospective lead role in a spin-off series that later became Highlander: The Raven (1998) where the lead role ended up going to Amanda played by Elizabeth Gracen
In this episode, the character Katherine has a partner named Nick. When the spin-off The Raven aired Amanda also had a partner named Nick Wolf.