Hello, I’ll be your beta reader for this story. Since I’ll be giving a close look at your later chapters, I thought it would be prudent to review the first chapter here. In short, there’s a lot of promise here in your grimdark vision of Pokemon, as certain elements are done really well such as the action, but there were a few flaws that held it back which I’ll go into later.

Generally, the writing flowed well, since you know how to build atmosphere with description, such as the dwindling bonfire at the beginning, and the little titbits that add context to Riolu’s past such as the dented rocks and trees. And for that matter, the fight scenes are great as every hit feels impactful, and adds to the dark tone you’re going for.

Unfortunately, a good writing style means nothing if you don’t thoroughly proofread your work, as errors are all over the place. There are spelling errors such as ‘uncounsciously’, sentences that should have certain words in them but don’t for some reason such as ‘that was the shortened that was told’, and lines of dialogue that use the wrong tenses, such as “How dared you?” It’s a shame because as I’ve said before, you have a good writing style, but if you don’t take the time to look over your work carefully for spelling and grammar issues to this degree, it can really take readers out of the story. I made a list of all the errors I noticed which I’ll send to you through a PM, just so it doesn’t clutter up this review.

Another issue I found is the characterisation, primarily to do with Riolu as our protagonist. To be honest, I didn’t find him all that interesting. The story is there as he sets off on a journey to join the Charmeleon’s gang, but we don’t know a lot about him aside from the brief snippets of his past. That would be fine if he had some charm or personality to make us root for him, but he isn’t shown to have any interests outside of fighting for survival. Even in a harsh environment such as this, there must be something for him to do outside of hunting for berries that gives us an idea of who he is. All I get is a weak Pokemon who’s waiting to be put out of his misery.

I have a few other nitpicks outside of that. During Riolu’s fight scene with the Machop, he stops the Machop from screaming for help by punching his teeth out, all with a remorseless ‘This is what you asked for’. That felt needlessly cruel and undiplomatic for a Pokemon who was trying to avoid confrontation just a few minutes prior. There’s also a brief moment where you head hop to Staraptor’s perspective, which doesn’t add a lot to the story and feels out of place. We could easily get the impression that Riolu’s emotional outburst was unexpected without the switch to her POV.

But despite these flaws, I look forward to reading more of your work to see how it develops. Even if your characterisation is underdeveloped at the moment, the writing style is good, the action feels punchy and the mystery surrounding the Red Arena is a strong hook. I hope you’ll take what I’ve written into consideration, and I would like to help you out more in the near future. Best of luck with your future writing endeavours.

[The fire that once brought warm to the cave he called home was gone.]

Warmth. You've got a lot of mistakes like that all through this fic. Proofread and look for a beta reader.

Semicolons should only ever be used when connecting two complete sentences and even then almost never.

[The nidoking may had been strong but in close range combat he was no rival for the terrifying strength of the four-armed pokémon.]

Poison resists fighting, though.

["What do you think you are doing?" Machop growled. Angry. But not as angry as Riolu.
"It was an accident! What is wrong with you?" Riolu took a paw to his head trying to suppress the throbbing. That was a strong punch Machop landed on him.
This sort of situations are the ones which lead to a fight. And this sort of fights are the ones who finish with only one pokémon standing. ]

You seem to be trying to do a thing about brutal nature, but in nature, animals don't usually get into fights for no reason. The machop is wasting time and energy attacking him and risking injury or death in the process. Hitting Riolu to get the other guy off of him would make sense, as would attacking him if the food Riolu's after is something the machop wants to keep for himself. But just getting mad at a stranger who's apologizing and no longer a threat is a terrible decision as far as long-term survival goes.

Also, when this is your introduction, it's hard to see anything meaningful about the Red Arena section. We see two pokemon battles in the wild. One looks like it's going to be to the death and the other only ends because a third party drives off the attackers. In that case, even if the humans are having pokemon fight to the death, they're not doing anything that wasn't already going to happen - and they're caring for the surviving pokemon which might actually make it less deadly overall since it means instead of dying by violence or starvation or sickness they only die by violence. I don't think that's what you were going for.

Really good job of writing the Pokémon characters they way you did. Lots of deep content in the characterization of them. Keep it up!