What was it that you wanted to convey with this character illustration? Is it supposed to be a piece of concept art that “explains” what this variation of the character looks like, or is this meant meant to be an illustration that portrays something about the character’s personality? For example, the body language and facial expression can convey a character’s personality/aura, and the environment can also help create a narrative context to gives insight into a character’s reason for existing.
Aside from that, there are some general issues that are visual art foundation problems, such as composition, lighting, anatomy, etc.
The composition basically is one that has no forethought into the overall visual design of the image. While an iconic composition of placing a character in the middle is a valid one, there are still room from creativity within that template, such how you offset that central focal point with background elements or lighting to create interesting tonal composition. Also, that raised canon arm is so close to the left edge of the image, creating unnecessary tension in the composition.
The lighting is inconsistent–you have rim lights that don’t show up consistently from top to bottom of the character, and other weird inconsistencies like the face being lit up brightly on one side but that nowhere else on the character is being affect by that same light.
In terms of anatomy, the mouth and philtrum area are too harsh in the turning of the form, creating harsh edges that should be softer. This is not a stylistic issue, but one of being able to depict the turning of form (the rate of turn of the curvature of a surface) properly.
There are some excellent tutorials and lessons in the Art Techniques & Theories subforum (linked below in my signature)–make sure you take a look (especially the sticky threads).