Redfall

All right, first things first - do not listen to the other guide and folks that say that this achievement is bugged.
The character trust achievements ARE NOT BUGGED.

The only thing that is bugged, at least for the time being, is the trust level display in-game (the hand icon visible in the lower left corner of the screen, below the character bars).
When you play in co-op with 3 other people the trust meter shows up +1 level after the first one, i.e. the trust progression is as follows:

Trust level 1 shows up in-game as Trust level 1
Trust level 2 shows up in-game as Trust level 3
Trust level 3 shows up in-game as Trust level 4
Trust level 4 shows up in-game as Trust level 4 as well!

There is a hard ramp-up in experience needed per level after level 3 so the progression between level 3 and 4 takes most of the first part of the game campaign.
Since the ramp-up between lv3 and lv4 is so harsh, and it is (currently) bugged and displays +1 level, people assumed it is bugged.

I can confirm it is 100% not bugged as I just unlocked 3 character achievements almost at the same time while playing in 4-player co-op with three fellow TrueAchievement Hunters. The trust level displayed lv4 for better part of our playthrough.

Our group wasted 3 hours of trying to do these achievements in 2-player groups, as we read the suggestions in other guide's comment section... Welp. At least now we saved you some trouble :)

To sum up - you will have to complete most of the side missions, safehouse missions and main campaign missions in the first part of the game (Redfall Commons) TWICE in 4-player co-op to have all five achievements pop for all 4 players. (1st playthrough - 3 character cheevos, 2nd playthrough - last character cheevo)


PS. Please be patient and do the main missions, side missions and the safehouse missions - it will pop soon enough and before finishing up the first part of the game :) Also, make sure you are semi-close to other players when you complete objectives - that is when you gain trust (points) with them.


Edit:
@KGI KlikoNL - The achievement ITSELF is not bugged, can be achieved normally with no issues other than BUGGED display of trust level in-game.
I have been in two separate 4-player co-op playthroughs where it goes from lvl1 to 3 instead of progressing from 1 to 2.

The achievement can be unlocked in 4-player session by playing the same amount of content it would take to unlock it in 2- or 3-player one.

Can be potentially buggy.

Play with a coop partner who is Remi, do not be the same character yourself as playing as the same character doesn't work with the trust system.

Other than that, just stay in vicinity of your partner and complete missions and get kills - seems to max out at 3.

I say it may be buggy as me and a coop partner played the same amount but theirs popped a while earlier.

EDIT: It may speed this up having a partner who is levelled up on your fresh start - I am yet to try this yet.

The red mist method took me exactly 60 minutes.

I did it with one other person because 4 player co-op was too laggy, glitchy, and buggy. This includes losing all the gear that I had gained upon traveling from Redfall Commons to Burial Point and having the trust level seemingly reset after the game crashed (more on that in a bit).

Basically, you or your partner stands in the red mist, and the other (standing outside of the red mist) revives the ‘downed’ player over and over until the achievement pops. My partner did absolutely nothing and we both got the achievement.

The green icon in the bottom-left corner displays your trust level, not the number of players that you're in proximity to. I know this because it went to level 3 and 4 despite playing with only one other player. For me the achievement unlocked several minutes into level 3, for my partner it unlocked once they reached level 4. This is because I had already gained trust with that character in other co-op sessions, whereas they were playing co-op for the first time. To summarize, trust appears to be cumulative even if it doesn't show up that way.

It's not bugged nor does it inherently display the trust level incorrectly. If you've never played co-op before, you'll need to get to level 4. If you have played co-op before, there's no telling at which point it will unlock, which is why people think it's bugged. If your game crashes, simply reenter the lobby and keep going. The trust level will appear to reset, but it won't (you'll just lose the ability to know when the achievement will pop).

There's also a trust meter in the menu (hover your character) that's made up of 3 bars. You'll start at 0 and the achievement will unlock at 3, whereas the aforementioned green icon in the bottom-left corner will start at 1 and end at 4. It's confusing, I know.