Like others, I'm pretty sure this was someone travelling on a staff travel ticket desperate to get home.

However, much as I understand and agree with the need to help our colleagues, I do think it was rather rude of this passenger to intrude on your space by putting their legs on the seat bar. As heckenhocker says, a small "sorry but we're staff and have to sit on jump seats" comment would've been nice. In fact, I think the crew should've told you this too.