There is an aggro mechanic and it seems to be based directly on your follower's max health, ranged combat stat, and melee combat stat. Certain weapons add additional aggro, with the highest I can see being the plasma rifle (it really should not in my opinion; this weapon is hardly my choice for fighting unless I want to decay enemy armor to capture them). If they are not wearing high tier armor (duraplate, HULC, or WEGA) and they have strong stats, they are more likely to be targeted too.
In my team (Kelly + Einar + Dexter/Milou/Aaron + Cassius), Cassius tanks, then Einar in the rare cases Cassius falls. The reason my Einar tanks over Aaron is that my Einar has higher combat stats total. (I did not train Aaron a lot in Ranged).
Ahh so that's why Cassius always tanks
Mr 220 strength...
Do you know if traits are considered at all in the formula? From what I've noticed Ayden seems to tank more often than I would think he should given the fact that his armor is better than Duraplate so you would normally think he'd be less likely to tank. If his overcharge thing counts then that would make sense. In my games melee and ranged skill aren't usually an issue for long since I train everyone on my A team up to 120 pretty quickly so everyone crits their way up to 150 in both.
Obviously in Ayden's case he's locked into his coil gun but for me he gets benched in favor of people who can use plasma fairly early. So everyone is using plasma+whip and everyone has roughly the same melee/ranged skill. So Casssius tanking due to his higher health makes sense, but I'm not sure why my combat focused wife is usually next in line in my games unless traits matter. She has less health than Aaron or Einar since she can only have 120 strength and yet she tanks before they do.
It takes awhile for everyone to crit their way to exactly 150 in both melee and ranged so it's been awhile since I saw what the effect is on tanking when everyone reaches identical melee/ranged stats but in my games they're rarely more than 10 points apart on the way to 150 unless I swap in someone new like Kelly that I don't have access to right away.
The other small wrinkle is that if using the group hotkeys to assign followers instead of doing it manually, whoever the AI puts first is going to gain skills slightly faster so if small skill differences are enough to make the difference then whoever is first might eventually end up as your tank. Cassius is usually the first to 150 melee since he's my capture beast. If I'm using Milou or a combat focused wife they're more likely to hit 150 ranged first since they're kind of useless with a whip against hulk armor.
I have actually had games where my combat focused wife ended up being picked first as the main tank even over Cassius despite him having WAY more health but I think in those cases she had significantly higher skill due to me using her from day 1 and not being able to capture and use Cassius immediately. She takes awhile to train up so that's not common though.
