If luthen was a jedi, as amca speculated (and I'm about 50/50 myself on the theory, it's compelling but idk), then I think Kleya might have been his padawan. It does kind of fit in my head with the relationship they have, which is maybe an argument in favour of the theory as a whole?
Kleya's actress is 27, and we're 6 years before A New Hope, which puts order 66 13 years before the show. If we assume that she's roughly her real age, she would have been 14 when it happened, which is old enough to have been a padawan...