Aye! Character design right up my alley! So far they are lacking in appeal for many reasons, but the first reason is because they look too generic. If you put them in a crowd they'd never stand out. Give your characters features that define them.
Like Chucky from the Rug Rat's frizzy hair and purple glasses, the Incredible Hulks overly muscular green body, Mario's red cap etc.
His head looks stretched and the shape of his eyes look pretty blocky and robotic which don't match his round nose. When designing characters you have to compare every feature from the feet to the head, and keep on redesigning it until you have a visual balance, try different shapes until it looks appealing. Also he's not very handsome, and comes across as ugly. He's ugly because his face is long like Squidward, his anatomy's off, one of his shoulders are way down compared to the other. Also many artists do it, but his eyebrows are not wide enough, for my own personal taste I tend to make the eyebrows a centimeter bit longer than the eye. I don't know small brow just look ugly to me.
Her head's off like others say, and she's quite frankly more appealing than the man. But her anatomy is off by a lot. She's lacking a forehead, her neck needs to be at an angle, and it's too wide. Her arms are shriveled up and should be as long enough to reach her mid thigh.
Also the style is very boring and gray, try to mix colors when making shading to give it more of a natural look, example if you have yellow you could just move the color wheel down to more default darker yellow color, but it would rock some socks if you picked yellow and add a darker orange in the shadowy parts of the character.
Here's your home work (if you choose to accept it) go to Google and draw people you see in the search, fat ,tall, petite, skinny, etc. Also before you make another picture, put the picture through a rough draft stick man comparing all of the anatomy features to one another in proportions. Learn how to draw an animation dummy, by adding standard shapes together on the stick man.
Also point perspectives are a game changer it really makes your backgrounds look way more realistic. Hoped that helped.