Great work! I will only say this because you are seeking some way of improvement without a direct technicality. Elements may be used to sharpen or defocus a character, even in abstract shapes. If a background has a square, consider if it has sharp edges or blurred. The sharpest element juts forward. The blurred element recedes. So if you wish to have a focal point, obscure what is not important with less clarity. My perspective is that backgrounds add a sense of gravity. When a background is simple, flat graphical shapes, it is more mentally conceptual, than grounded. So the shapes drop down lower to the base of the brain instead of the forward lobe. circles are friendly, squares are sturdy, and triangles are to be cautious of because they are sharp. You can take those primitive commonalities and juxtapose them or work directly towards them. Shadows ground a character because it defines a plane on which things rest, which is a shape itself; often a square planar (with no end if it is a horizon line), unless you show the rounding of the earth at certain heights or out of choice of style. So that is the gravity I speak of. It is a sort of "placement" into a zone that can be defined. Even a blurred ambience underneath implies a plane on which the ambience is projected. To have no background means you are dealing primarily with shape of anatomy. I would advise studying into the nature of why our bones are shaped the way they are. Gravity imparts a lot because we are geometrically opposed to the center of our spherical earth. Many character compositions are triangular in Leonardo Da Vinci's work, as well as Frank Frazetta, because the triangle itself is a stability against gravity. You can invert it to immediately become interesting because logic says that a triangle on its head is less likely to stand on its own, so the base of the brain observes it for potential changes in direction of falling. The above statement is a very lengthy study and reaches deeply into physics and basic survival elements related to martial arts and even mathematic triangulation (why things fold inwardly in deformations as triangles in animation) so it is not a study to be taken lightly. It may be quite absorbent, so get into it to gain something out of it without falling too deeply into the infinite rabbit hole of the Universe. One last thing to consider in posing: Martial artists are animating themselves at all times to cantilever and create force. Once you understand such posturing it is easier to decide to be more defenseless or more defensible. (often done to feign an opponent into false-security or baiting them towards an opening that they intentionally set as a trap). Style of movement is sometimes defined by say... a broken wrist or a catch in the hip. Every person has some level of difference in movement. Martial artists often employ overall body shapes (think of the song YMCA and the dance which mimics the letters). It can be a great study to consider how this anticipates themes that are being presented in the work. Is the cowboy off kilter or his gate wide and stable? Is the kid frightened of the monster into an unawareness of their own stance? etc. This will help compositionally because the pose will not become so straight-forward. You can alter what you see fit to add towards such clarified thematic in the storytelling of the image. You can adjust things without background to have an internal rhythm instead of it being defined by the external but the character will begin to float the less it is graphically flattened do to the elements of gravity being less easily understood at the immediacy. This can be intentional though and convey floating as a theme with intention if you so choose. Good question! It is hard to verbalize and I love that you challenged to ask it! Your work is colorful and very tasty in its color choices! Keep up the good work!

Not all drawings need backgrounds unless it is necessary to give context to the figures. For example: the second image of the girl on the horse, is the horse flying, jumping, are the front hooves on the ground? I think even if you used a shadow shape, it would help give context to the image. Other than that, I think the rest work well.

Great drawings!