I know a hero only takes a hit when a star is in a fire card, but can he "protect" the unit he is with? Or would that unit come under fire as normal; on cards without stars?
Units that have a hero marker are hit as normal (ie a Hero can't protect them). However if it is a single step unit you can choose to sacrifice the hero (flip it to the "inspired" side) to keep the unit in play.
Note that Hero, General and HQ units are only chosen to take hits after assigning hits to all other eligible units.
See section 11.1 and 11.4 for more details.