Jillian and Marika were lovers.
Marika was an early, if not the first, lover that Jillian had. Their relationship was intense but started to spill over into their work in the field, which lead to problems. Lover's spats became insubordination, and in the end Marika ignored Jillian's orders and was croaked at the Battle of Blitzkreig Bop.
This was Jillian's first lesson in the importance of keeping her work and personal lives separate, though not her last.