It's a bit hard to say. Is English not your first language? Your sentences felt awkward. Take a deep breath, breath in and out. Then steel your stomach. If necessary, put a bag of ice on your head in case Ai-chan's words start to heat your head. That being said, do not let Ai-chan's words make you stop writing. Ai-chan is only mentioning the biggest offenders. Ai-chan will only comment on chapter 1.
Yuuma's stomach was grumbling, having overslept he had to miss out on breakfast.
>> While you can say that his stomach 'was grumbling' (past continuous), it lacks the immediacy of the situation and makes it feel detached. If you say "stomach grumbled", it would instantly feel more attractive, to say the least. "He had to miss out on breakfast" also feels jarring.
>> If Ai-chan were to edit this sentence, it would be "Yuuma's stomach grumbled. Having overslept this morning, he ended up having to forgo breakfast."
"Looking into the small plastic bag in his hands laid one easy to eat ramen pot, he grabbed the quickest thing he could find at the local supermarket."
>> What was this supposed to mean? Correct Ai-chan if Ai-chan misunderstood, but Ai-chan guesses that he went into the supermarket, grabbed the first thing he saw, then he sees the ramen pot in his plastic bag? What is a ramen pot anyway? Ai-chan is Japanese and has never seen a ramen pot sold in the supermarket. Google search suggested that it's ramen cooked in a pot, but those aren't sold in supermarkets.
>> If Ai-chan were to edit this sentence, it would be "Looking into the small plastic bag, he spied a cup of instant ramen. He had grabbed the first thing he laid his eyes on at the local mart."
"I wish I had breakfast before running out the front door, metaphorically a cloud of doom and despair surrounded Yuuma."
>> Sorry, Ai-chan doesn't know how to fix this without completely rewriting everything. Ai-chan assume the first part before the comma was supposed to be the monologue, in which case, you should make it clearer, maybe use italics. The second part is really awkward. Maybe just get rid of 'metaphorically' and rewrite everything. We know that people are not literally surrounded by cloud of doom and despair in real life.
"Her cheeks started to turn red as she avoided eye contact with him, "S-Sorry Akiyama Yuuma"."
>> "Sorry Akiyama Yuuma" is not what a Japanese person would say. Nobody calls people by their full name.
>> She would say, "Sorry, Akiyama-san" if she's just someone who happens to go to the same school.
>> "Sorry, Akiyama-kun" if they're in the same class or at least on speaking terms.
>> "Sorry, Yuuma-san" if they're close enough to call each other by their given name but still maintains a distance as to address each other respectively.
>> "Sorry, Yuuma-kun" if they're close enough to call each other by their given name, is close enough to be maybe something like childhood friends, but not close enough to be lovers.
>> "Sorry, Yuuma" if they're very close like lovers or have familial relationship, such as being cousins or so.
"Arriving before the front gates of Seiocha High School, Yuuma was prepared to tackle like any other."
>> Tackle what? And Seiocha can't possibly be Japanese.
""Never would have thought the mystery girl was you, Tomohime" Yuuma nervously laughed."
>> Ai-chan needs to inform that Tomohime is not a surname. Tomohime is historically a nickname, Princess Tomo. It can be a given name, meaning 'Rough Princess' but it can't be a surname. While there are new Japanese surnames registered by foreigners, most Japanese people carried their ancestor's surnames from centuries before, with deep philosophical meaning. Ai-chan's surname for example, 椎名 (Shiina) roughly means 'Distinguished Oak' but it can also be elaborated as "a strong, unmoving beauty whose feet holds strong against the wind".
"She waved her hands innocently, "It's fine you don't have to answer now if you don't want to" Mitsuba starts to walk away when Yuuma suddenly shouts to her."
>> This is probably unintentional, but you do need to watch your tenses. If you start with a past tense, you're usually talking about something in the past, so 'starts' here should be 'started'. Sure, there are exceptions, but there is no exception here.
"Oh shit! I can't stop myself I'm moving too fast!, Yuuma's momentum was still sending him over the edge. As if the slow-motion had taken over the next few seconds seemed like a lifetime."
>> If this is a Japanese school, this would not be possible. All Japanese school rooftops have fencing like the one below. It is to avoid students from committing suicide from jumping. Sure they could jump from the second floor, but it rarely results in death. In reality however, very few schools allow students to go up on rooftops. Rooftop doors are usually locked and sometimes padlocked.
That being said, this image is also inaccurate. Japanese schools rarely have air conditioning. And those schools that do have air conditioning wouldn't put a single air conditioner on the roof. Like what is it supposed to cool with just that one? Some newer private schools do have air conditioning, but only for certain buildings or locations such as the library or the school hall. Ai-chan only uses this image because it clearly shows the fences.
