Ever since I started drawing 10 years ago, people say my characters are stiff, I don't know what to do about it... 
Maybe show me the characters of that page with "non stiff" pose?
I'm not much of an artist but I have a question. Why are their legs exposed? (They are right? I don't have my glasses so I'm not sure XD) Won't that be a handicap in a fight? They could aim for their exposed skin and the like.
gesture! geeeeestuuuure!!!!!
im not feeling up to explaining things rn, i dont think id do a great job at it, so im gonna link to some great advice i got about a year ago now
at first, understanding when a drawing is stiff can be really hard. but as you develop your gesture drawing, itll become like second nature. its something you can really get a feel for with time.
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also, i think you could do with some studies on how long hair works. the way you have your long haired characters hair, it looks very shaggy, and not representative of how long hair works. most hair will all be the same length, with one or two off flicks - when drawn like you have, it looks like they have a tail attached to their head.
your shading on the armour in that splash panel is really strong, but elsewhere you could do with higher contrast and less white space.
I tried several types of hair for the woman that appear in bandages, this one is the only one I found to be "less generalist". If I change her hair a single bit, she'll look totally different...
I really like your style! I'm a big sucker for fantasy armor and weapon designs, huehue.
I think I have to agree with @HollowEve that the necks are a bit too long, and their heads are leaning too far away from the body. Here's a quick reference I found on twitter that might be helpful.
Man, drawing humans is a pain, I didn't get a single complain about the Forest 
Not so long ago, my necks were too short, now they're too long 
Anyways, this is not my main comic. this is just a one-shot I wanted to draw. I think the clean style makes my mistakes stand too much, I think I'll go back to the sketchy style...
The more armor, the most restrictive the movements will be, If you can't evade, you'll get killed even if you're covered with armor from feet to head
And their legs aren't exposed, this comic is inspired by Claymore and Berserk, ´nough said
y'all need geeezture draaaawiiings
false!, look up some medieval armour videos, full plated armor is a lot less restrictive and more flexible than movies, or manga in this case, would lead you to believe =P
*studies too many armor videos for his comic
in fact if it was that restrictive nobody would use it, since they could not move effectively or quickly in battle, and believe you me in real medieval combat speed and flexibility is a must
Before I moved, I went to some life drawing sessions at the art center in town. We did long poses as well as gestures. It's incredibly helpful. You'll see a huge difference in your artwork because of it.
What @IdiotWithPencil said. Armor has been used for centuries and contrary to popular belief, if you've fine through enough training for them to even hold that sword beside them, full armor won't be restricting at all. Also, I don't know much about your comic but Berserk definitely had fantasy elements and the guy was boss so I can't really tell if your character would be like that. Was only asking from what I was seeing.
yep =P between two people who have comparable skills if one is completely armored and the other one isn't the one with armour would win 9 times out of 10 if we are talking about swords of course, if she had a hammer and shield that would be another story (hammers cave plat like butter and the shock is absorbed by the body)
Well, this is a supernatural fantasy comic, considering the forces involved, armor is purely decorative, the majority of the other characters don't even use armor.
Look at the woman in bandages, if she was in her top form, the woman wearing the armor wouldn't even have time to draw her sword
im not sure what you mean by 'less generalist', and i disagree tbh. can i try an example?
vs
dyou see how it's the same long curly hair, but with less flicks and a little more looseness it looks more like continuous strands?
I know that feeling too well xD first i did too smal noses and now they are too big... But don't worry and keep on drawing and improving. Even though its very frustrating sometimes you still will improve. Look back on some older stuff and you will see 
btw. I really enjoy the forest and how balanced the details are.
So I think you mostly just need to brush up on proper anatomy. I get the feeling you've either learned largely from looking at other drawings or you just haven't spent enough time drawing realism. In addition to gestures and life drawing, I'd suggest taking the time to draw skeletons, muscles on top of them, then the skin on top of that. That sort of break down can go a long way in teaching you how things work.
The reason your pic here looks "stiff" because the pose isn't very natural for sitting and the weight distribution is off. You kind of have an in between going here--to look natural, the person should either be leaning back or forward more. Here's the example I drew:
Personally I think 1 looks best. A person's center of gravity is at their core (hip area, about) so in order to balance, their weight has to be distributed across whatever limbs they're using.
The other thing I noticed is that the armor doesn't... look like armor? You have lines that kind of insinuate it's supposed to be armor, but there's no structure to it. It almost looks like they're wearing a shirt with lines printed on it. D: To fix this, you have to make it look a little more rigid and give the plates proper space on top of each other, like this:
Rather than JUST thinking about the design, also try to imagine how it wraps around the body and even how it's constructed or held together. It'll go a long way in making it look more believable! That goes for regular clothes, too.
I would have to agree with @revisionstudios, I see the same thing going on with the greaves. Here is pics of armor7 for reference because I can't show you what I think is wrong as i'm not an artist.