[Mivereous]

PvE enemies seem to be more focused on Player Characters than 'Others' (Mercs/Shine Knights/Summons (Youkai/Butterflies)/etc). It's almost as if the 'Others' have a psuedo-Incognito; if an enemy is in range to attack an Other, then it will attack, then move towards the nearest PC. The only example I can think of this not happening is in the instance of the

Ashe/Seto/PC fight, in which Seto seemed eager to attack just whoever was closest, rather than focusing on the PC, in my experience.

[Ranylyn]

As someone who extensively plays Engineers and Summoners, and to a lesser extent priests, I am 100% Confirming.

Enemy Behavior against:

Engineer Bots: Enemies will often be content attacking one they start beside and focusing on it, but otherwise run circles around them to exclusively aim for the player.
Youkai and Shine Knights: Will attack them if they start their turn beside them, but then move and try to target the player anyways.

In all cases; if cornered by allies or enemies, even ranged PVE mobs like bandit gunners will not bother getting closer. This can be exploited to limit the number of threats at any one time.

Similarly, Shine Knights are obsessed with always moving if they are not directly adjacent to an enemy, even if the enemy is within range, which leads to a player who needs their protection playing a frustrating game of tag as the Knights and PVE mobs take turns moving and forcing the player to keep pace.

[Neus]

Associates have a lower priority than players. Mercenaries are associates. Ergo...

I could see mercenaries being given a priority similar to players since you can't continuously summon them in battle.