So it's a nice short entry chapter. I can see the nice relation, supposedly, between Kira and Viv. It's a bittersweet rivalry, and I kind of like to see those more often than not. But it could just end up with them traveling together. Who knows? You do. But I don't, so I'll have to find out later.
You introduced Flint in here, basing off that spoiler tag with "Character Descriptions", and be sure to explain why he's there. I bet you could have used some other character, preferably from Kanto, such as Lance or someone. And going on with the character spoiler tag, you don't really need it. I mean, we don't really care how tall Viv is, or her eye color, etc., unless it's relevant to the fanfic. If it's not related to the plot, then it's just kind of like candy, too much will make you fat. Okay not really, but you get the point.
Also, characters are better described by their... well... character. How they act is what defines them, not their hair color. So keep that in mind for future developments.
Pouting, the girl would return her Nidoran to
it's ball.
The bolded part should be "its" since that is the form that represents possession. "It's" is the conjunction of "it is". This little grammar mishap happens a lot more further down the chapter, so be sure to brush it up a bit.
His right eye would twitch as he gave a command, “Mankey, use Sand-Attack!” Jumping down from the boy's shoulder the Pokemon's body would move swiftly; Landing onto the ground, it would scoop up some dirt in it's hand as it charged towards the Spearow, tossing it towards the other Pokemon's eyes once it was close enough. Spearow would begin to flap it's wings rapidly in a blind rage, attempting to gain the upper hand by getting higher up. “Use Scratch now! Try to keep it close buddy!” Before it could get even a few inches off the ground, the Spearow would be knocked right into the ground. Mankey would be gazing down at the bird-Pokemon intently; Awaiting either orders or for it's opponent to retaliate.
Right here, it's a massive block of text. That's actually not problem too much, but when you have different sets of dialogue spaced out with a huge description in the middle, it's better to keep the next bit of speaking on the next line. It's just neater that way, and you stuff in dialogue smack in the middle too.
For a brief few moments neither side budged. That is until Spearow let out a high-pitched cry.
With their eyes wide, the boy would quickly grab up his sack, his Mankey scooping up dirt in it's left palm as it clambers up onto the boys shoulder.
For the bolded part, the "their eyes wide" would actually refer to the boy, which is not what you're intending on. You want to explain the Spearow, so you can add "with their eyes wide" to the end of the sentence before it, right after "pitched cry" and comma. Or better yet, stick the bolded part in front of the previous sentence so it's "With their eyes wide, the Spearow let out a high-pitched cry."
Further down the road the pair could see Viv;
Her arms would appear to be crossed as she kicks a rock as she walks. Not but a moment after she came into sight did the sound of many flapping wings reach their
ear's.
You have a semicolon, and the "Her" doesn't need to be capitalized since the semicolon makes it all technically in the same sentence. If you wanted to keep it capitalized, simply replace the semicolon with a period. As for the "ear's" it should just be "ears", since "ear's" is showing possession and there is not object for it.
She'd turn around to him with a gaze of anger directed towards him, “KIRA! If you even spe-,” as she had begun shouting,
"She'd" is "she would", but that's inconsistent with your tense. The correction would be "She
turned around".
In a matter of moments Kira had caught up with her, his Mankey resting on his left shoulder with an arm
would the top of his head.
Yeah, I don't think "would" is the correct verb for this. Try something like "around".
Glancing back for a brief moment
he'd see a man wearing a red-wool hat,
dreesed in baggy cargo pants and a faded yellow shirt walking towards the flock.
Bit of a typo here, the correct spelling is "dressed" with two s's, not two e's. Also, same circumstance as mentioned above with the "he'd see", it should just be "he saw", again following the tense. This happens more as you go through it.
He'd come to a stop to turn back around – His mouth opening to shout just as the man sent out a
Ryhorn.
Probably a typo, but I'm going to point it out anyway. It is spelled "Rhyhorn".
Cries of panic would echo throughout the area as the Spearow
turn away in fear; Flapping back to the east where the had just come from.
You used "as" in this sentence, so it should be "turned", again agreeing with the tense.
As his head rose up to
back look at where the man had just been,
Switch the two words around so it's, "look back".
There were various
tree's lining the sides of the road as well as plenty of boulders the figure could have moved behind.
Trees. No apostrophe.
So that's the majority of the spelling/grammar mistakes. A lot of them were repeating. Also, try to vary your sentences a bit. I saw that you used "As he was..." (or similar sentence) a lot. Keep it fresh, keep it new, keep it exciting.
There's not much else I can say since this is only chapter one. I'll look forward for your future chapters and keep writing!