I am going to apologize for if this comes off as harsh. I am going to try and be as constrictive with this as I can.
One of the first things that my eye was drawn to when I look at this is the line work/line dividing the picture in half. The line work is a nice consistancy that I do enjoy seeing for a more cartoony look. The only thing that i really have to say is that it is a little rough and wobbly which I can attribute to hands shaking, especially if this was done with a mouse. Which Low key here drawing with a mouse is hard and you have my kudos for that.
The second thing that I notice is that there are a few small sections where the color from one section bleeds into another like the purple from the Frisk side of the drawing bleeding into their hands. The coloring itself is really really nice, and I do enjoy it. The one thing that I can say is if you have photoshop or Sai is to use layers to kind of alleviate with that bleeding. A free tool I can recommend if you do not own either is pixlr, it is a free online version of photo shop and good for learning how layers work.
The only real complaint I have so far is shading, I don't see any real shading in the picture that gives me an impression of the changes in lighting like if the sun is shinning through a hole onto firsk or darkness is causing shadows on Chara's face. A really easy way to do that is to take the skin color and just darken it by one shade and doing a small outline.
All together the piece is really nice, and I like your style of drawing I think that if you put those small things to use it will totally look amazing, It will be interesting to watch you grow and mature as an artist. I am rooting for you.