Ah, a hopeless underdog Trainer seeking to beat his personal rivals, all of whom are infinitely better than him. This could be interesting.
I'll begin, as I'm wont to do, by stating the major points that need work in the story and work down to the minor ones. Firstly, I have an issue with the structure. The chapters are so short that they don't really feel like chapters, and although plenty of significant things happen in them, you don't dedicate enough space to each point for it to hit home effectively. For instance, that battle with the Rattata? It's his first battle. It should be treated as such - as something worthy of at least a little more than the cursory description it got.
Chapter Two, where Marcus muses on why he's become a Trainer, is also something of a structural anomaly. Why is it placed where it is? It's strange that we have him and his Cubone fighting in the first chapter, acting as if you expect the reader to have a full grasp of the situation, when in fact there's been no set-up at all. For the full effect, we'd want there to be a sense of anticipation - and yet the part that would have helped build it is to be found in Chapter Two, after the fight. It just seems oddly laid-out to me.
Secondly, and perhaps because of the first thing, the chapters don't end strongly. I mean, you obviously have an instinctive sense of where to end a chapter - you end them on the right sort of event and tone - but the endings themselves aren't that well-expressed. A chapter ending ought to have more of a sense of finality to it, as well as a forward look to the next one. Let's take an example.
Maybe this could work out, or at least, maybe he could get Cubone to cooperate.
This is the perfect event to end on, but not the right sentence. If I were you, I'd end with a shorter, punchier statement that really catches Marcus' reaction and lets the reader know that something's definitely changed in their relationship. If you'll allow me, here's my suggestion:
Marcus stared after it, and gave a short, incredulous laugh. Maybe this could work out after all.
It doesn't have to be like that, but your chapter endings should leave us feeling both like 'Mmm, that was a good chapter' and like 'What? It's over already? But I wanted more!' at the same time.
OK, those were the big things I noticed. Now, I have some more minor points - easy to fix, and therefore good for making work better very quickly.
You mean 'were', not 'where'.
OK, this is kind of strange. Why is the text bolded? I can't work it out. It shouldn't be. Also, you mean 'said' rather than 'spoke'; 'spoke' can't be used when someone actually says direct or indirect dialogue. You can only use it in the same way you'd say that someone 'stood'. For instance:
In other words, 'spoke' cannot link to actual dialogue.
Why is Adrian so nasty to him? I just don't get it. It's like he's grabbing at any excuse to stop Marcus from being a Trainer. Half of these aren't even real reasons - for example, why would people take him as a joke as an older Trainer? It'd seem to me to be a more sensible course of action to take, and therefore that people would be more wary of his strength. In addition, what's all this about 'childhood innocence' being necessary to raise a team? Why would anyone think that, ever? It just... doesn't make much sense. If you're going to state that only younger kids ever become Trainers, fine, but the reasons you've cited aren't the most believable. I'd make more of the whole 'you'll have wasted your time in school' bit, if I were you; that's the only strong point Adrian makes. Base his argument around that and it becomes a lot more believable.
You're missing an 'arrived at' or something between 'eventually' and 'Viridian'.
Commas cannot join two complete sentences together like this. It's called a comma splice, and while some authors consistently get away with it because they happen to be John Updike and José Saramango, we mere ordinary writers have to respect the rules of the English language. You could either change the comma for a full stop and make it two sentences, or replace it with either a dash or a semicolon. I'd recommend the former - it'll read better.
Right. Now I feel very guilty about having essentially poured cold water all over your nice warm story - but I always think that if a story can be made better then it should be made better, and I hope I've managed to be more constructive than straight-out critical.
Anyway, good luck with this and I'll be sure to check back to see your progress!