These are looking nice! The biggest issue is that your captain has no groin  The groin should be slightly below his jacket level. And the legs should pivot at a hip location a little bit lower than they do presently. You could also nudge the jacket up a little bit higher if that makes his legs seem stubby.

In addition since he is on an angle toward us his shadowed foot should fall and lift slightly further forward than his lit foot when he steps. Currently you have it the opposite way around.
Keep it up

This is looking great already! The overall timing is perfect 

The way I would add more power is to make sure that your build up happens in the opposing direction to the ultimate motion. So ideally you want the figure to squish downward before springing up. And then you want to have a longer buildup of energy, keep squishing a little more every frame, really ease into the final pose, and then hold that final pose for just a moment before releasing.

And I would take another look at the feet, having them spring in and out makes it appear to release it's energy early before the big motion. Have everything moving in one direction for the whole duration of the buildup.

As for the effects you really can't overdo it. Use lots of frames, lots of little tendrils and flecks of light that slowly fade away. Especially for the launch. Rather than having the effect move upwards like it does try having it dissipate in place (i.e. make it thinner and break apart a little frame by frame until it is gone)

Overall I would say the biggest thing you want in your timing is contrast, you want the buildup and the fade out to linger as long as you can and have the motion happen in just one frame like you have now.

Good luck 

It's difficult to tell what you're trying to achieve here. Seems like you've got the character moving which is a great start though! I think if you have a clearer plan for what motion you want to achieve you're going to find yourself a lot more inspired with how to proceed. What exactly is it that you want the character to be doing, and what are you trying to communicate with that motion?

I'd also recommending thinking about all three dimensions when you're animating. Things like the pelvis, ribcage and clavicle rotating in 3d space will add a lot of appeal to any animation.

The character is a pretty complicated choice for 2d, and especially pixel art. All of the small bones in the hands and feet are going to cause you a lot of headaches. What is your reasoning behind this style? Is there any way you could go back and consider an approach that is either more suited to pixel art or a technique more suited to this size and shape of the character?

Looking forward to seeing how you go ahead with this 

These are looking really good! I wouldn't be too concerned about them looking too similar to your references, I didn't peg them as tactics advanced.

I think the biggest issue you're having is with your colours. I think you should try using fewer shades per colour. For a sprite this size it's creating a lot of muddiness. Consider using fewer shades with higher contrast.

The outlines on the boots in the first sprite being lighter doesn't look very good. And generally I think your outlines could be a bit darker.

Try to avoid having big staircases of differently coloured pixels like you see on the mermaids tail (called banding if you would like to learn more), instead try to use bigger clusters of pixels to describe the shape.

Be very careful with small details like the green in her hair and the spots on her scales. Having single pixels or very small patches of distinctly different colours can look like noise. Try making them a bit bigger, exaggerating details like that is often necessary in pixel art.

I love the idea for the goblin enemy Keep it up this is all looking really good, a little more polish and these are solid assets

Wanted to clarify that animation in a small number of frames is certainly possible! Just that if this piece is intended to help you learn animation techniques that challenge is going to severely limit the types of things you can do. A higher frame count, or at least not using the idle frames, would serve you better in terms of understanding what techniques you are using and how a walk animation works. The better you understand those things the easier it will be to make a convincing animation under a set of strict limitations.