Awesome job here. I can't say I know anything about the subject matter, (don't play Splatoon), so I'll critique based on what I can see.
--) Right off the bat, I love the use of colors. These particular shades of green and pink look awesome against each other, and the glow effect around the edges of the characters is fantastic! It makes the whole piece so vibrant, and makes for a lot of visual interest. I also really like Pearl's hair.
--) The shading is very subtle, but it works. The only complaint I have about that (and in the whole piece in general) is that the light source doesn't look consistent. For example, on Marina's arms, there's reflected light coming from both sides? There may be a reason for that, and forgive me if I'm missing it, that's just what I noticed. If I had any recommendations on the shading, I would tint it either green or pink, depending on the side they're on. I think that'd look great.
--) The composition is very solid. It's very clear that both characters have the same amount of importance, despite Marina being a bit taller. Both characters are centered correctly, and the background is nice and subtle, not overbearing in the slightest.
--) The style is great. I'm a huge fan of the toon, stylized kinda thing. It's toony, but it's clear you also understand the fundamentals of drawing 3D objects in 2D space, like in Pearl's crown, Marina's tentacle... things (idk what they're called), and the zippers on both of their outfits. You obviously had a style in mind, and I think it turned out really well.
All in all, I love this drawing. I may not seem that credible, considering I've only got one piece of art on my page so far, lol. But I am new here and looking to grow myself.
But yeah, you've got some awesome talent on your hands, and I've happily reccomended you to the portal ^_^