Here's my original comment: "I love the character, but there are quite a few times where the character's mechanics clash with & break the balance of other mods.

In the Ruina mod, there are a few fights where you have to give allies some of your shield to protect them. This mechanic doesn't work at all if Mon3tr is alive. (You can't give them your shield because "you" don't have any, Mon3tr does. I know there are a few cards that do give you shield, but it doesn't matter if you don't have those cards.

In the The Jungle mod, there are a few fights that get messed up because you can't target enemies with attacks (except for the special operative cards) (The Spider boss, the lion, and the mushrooms)

There are a couple more examples that I've come across, even though I've only played this character for 4 runs, so I'd say that it's pretty common.
One of those examples is that atleast one of the special "ego" cards that you can get in Ruina just doesn't work at all, since it applies a damage-altering affect to you, when Mon3tr is the one doing damage.

Another is that there are many enemies that apply debuffs to you, but effectively do nothing because you have Mon3tr. (for example, there are many enemies that directly give you vulnerable, but this barely even matters since they're only attacking Mon3tr)
There are quite a few buffs that you can get from relics & events that are rendered useless due to this same problem.

There was another example where an elite enemy almost killed me because it was designed around a special buff that makes it super strong, but is disabled if you attack it 3 times in the same turn (which is almost impossible with Mon3tr) I'm pretty sure it was the Lion from The Jungle

I feel like many of these problems would disappear if you and Mon3tr shared the same debuffs/buffs. (maybe creating certain special buffs that disappear when Mon3tr dies, and only affect one of the two characters, so that you can have both "Mon3tr strength" & "Kalt'sit strength" or "Mon3tr metallic" & "Kalt'sit metallic", since they work slightly different depending which character has them.)
I feel like many of the remaining problems would disappear if you could choose who to give shield to. It wouldn't matter very often, but when it does matter, (such as during Ruina ally fights) you'll need it.
Making Mon3str be able to hit multiple times more easily would also help.
Making more cards that target specific enemies would also help. (such as, making a card that works the same way as the one that commands Mon3str to attack the lowest health enemy & adds 10 intent, but instead have it target a specific enemy (enemy 1, enemy 2, enemy 3, etc.) and add 8 or 9 intent.)
(this suggestion wouldn't even need you to rework how the character fundamentally works)
I know these changes would require a lot of work on the code, so nobody's gonna blame you if you just leave it the way it is."

I originally had to cut this comment up into two parts (and cut some parts out) because it was more than 2000 characters. (steam limits comments to 1000 characters)