If a skeleton is in defensive mode, it should only fight mobs if either a player attack it or it attack the player.

"Defensive Mode" is not really a concept in Overlord 3, but I do plan to allow them to do something similar to what you're after.

Hmm. Perhaps not with current defensive mode, as that is intended to defend your base from mobs, but I may add another mode with this capability. If I do, the current Defensive mode may be called Defend Area, and the new one would be Defend Player.

I've been using skeletons as bodyguards in the Erebus, using Follow + Defend. They're a bit like a herd of excitable puppies chasing after things 40 blocks away that aren't at all interested in trying to kill me.