Hi there!
I'm impressed with the Tapu Koko art - it's design gets a bit lost, but it also wasn't made for the size you're working with so I have to say it's the best looking sprite! How long did it take to do? :o Nice work!
For the others, they aren't as impressive as that Tapu Koko sprite. :c You had provided a lot of design information and it was pretty great as a presentation. The trainer back sprite is alright, but the shading needs a more defined light source - the collar of the shirt looks weird because of how it's shaded compared to the shirt and hair. It looks out of place rather than part of the outfit. I'd almost suggest switch the values in the collar? It would have a little bit of light on it, so you'd mostly want the shading color overpowering the lighter color.
For the shirt outlines, I think there needs to be some spots of black. Possibly around areas that would have the most shade and separate outlines? Example, like the arm sleeves (i circled maybe ideas where to add some black outlines)

And finally, the Celebi!
It has weird outlines? And a very tiny neck :C It's also a green sprite over a green background. Part of the body gets lost in the color but also it's not very exciting to look at. I think the background is great! It has awesome detail, you can read the title fine, but the Celebi has similar colors. If it was the shiny palette, it would be a different story! That's because pinks, reds, and purples compliment greens better! They're on the opposite sides of the color wheel:
I know purple is opposite of yellow, but purple or magenta work fine with greens. I'd keep it in mind even though your title can be read quickly and its understandable, make sure the sprite and background don't get lost or fight for attention! Also the weird open space in green is kind of. I guess empty? Probably means you can move the text or art around. Anyway, what's the most important part after the text? That will help answer what you'd want people to look at next.
Overall, nice work!