Beetlethief

I really like how you joined together characters from different worlds together into one. Beetle Juice steeling the lovely jewel from the Knuckles!

So it seems that you are trying to tell a story here. Is Beetle Juice steeling the jewel? If he is than Knuckles should be putting up his fist and looking toward Beetle Juice. This will better express that Beetle J is steeling Knuckles jewel and knuckle's isn't happy about it. Right now Knuckles seems to be just looking off somewhere to the left.

Nice texture with the grass and clouds.

Now one thing is, if you don't want your characters to blend in with what they are wearing you want to make sure to use different colors to make them stand out. So in Knuckles case, his red body almost seems to be the same color of his shoes and Beetle J's eyes seem to be the same color as his hair. Just by changing the colors by a shade or two will really help separate the characters with those similar colors I mentioned.

Now if you want to keep the colors the way they are then value, lights and shadows will really help separate everything. Which brings me to lighting. (I may be getting a bit more technical than what you were going for so if I am take my comments light to heart)

On to lighting... if the light is coming from the upper left (which I'm assuming based off of the highlights on Knuckles hair and shoe) then the highlight on his hair strands would actually be on the opposite side. Instead of being on the right side of his hair strands, it would be on the left side of the strands.

Beetle J who is holding the jewel would have some reflection of that nice shiny jewel on his clothing. So there would be some green on him. This green light would emphasis that light is shining on the jewel, that the jewel is shiny, and that he is holding it close to him. Think of when they show someone opening a treasure chest, how the person's face ends up glowing with the color of whatever the treasure chest holds. So if it is gold coins then the person's face usually has some gold light on it.

Now you have very nice clean lines.which is great. Many people struggle with nice clean line work and you did a really nice job.I like your idea behind this! I can totally see Beetle J as a thief haha