I'll spoil as much as I can get for the reason why fatso is mad at maro and how the rest of the tournaments gonna go
Nanzan - Maro
Fatso Minamiya is one of Maro's original group he's also a loser who wanted to be one of the popular kids. He proposed the idea of becoming an adventurer to his friends but they all just laugh at including Maro, his reasoning being that their is no way he can procure money to buy cards and that they are loser otakus so there is no way for them to become an adventurer. Minamiya pissed cried to his parents to buy him cards to become an adventurer and they bought him the safest and cheesiest deck the proliferation/summon deck it's a deck that works well on early stages and low rank dungeon as there is literally no strategy than just overwhelmed them with numbers. After becoming an adventurer Minamiya become popular as expected and Maro seeing how Minamiya manage to brute force his way into the school caste and the fact that Minamiya is now closer to the girl he like decided to also become an adventurer out of pure jealousy. But since his family is not as well off as Minamiya is he is forced to work multiple part time jobs just to be able to buy cards and he even gamble it by literally choosing to roll the gacha lmaooo. Minamiya on the other hand becomes complacent and he really never tried to conquer the dungeon and the feeling of superiority over his old friends made his head ballooned out of proportion. Maro on the other hand got his ass handed to him and got fucked in a beginner dungeon with a unique encounter because Renka decided to play with fate a little bit.
Renka Spoiler
Renka is a Special Cursed card called a Key Card. Curse Cards are cards that had the power to use their abilities and affect anything on the outside world. Renka didn't match well with her previous owners so she developed a debuff that cuts her power output. Not wanting to repeat the mistake again she subconsciously manipulate fate so that she will be sold to the person along with the cards that are the most compatible with her that is why she got good chemistry with Maro, Eliza and Yuki. She didn't know about this power though so she pretty much clueless but she already got a gut feeling about it. This comes with s price though by manipulating fate towards his side it will surely comeback as misfortune one day due to the rules of causality 'what goes around, comes around' and it turned into an irregular encounter manifesting in a low rank labyrinth.
About the Skill: Falling Existence and spoilers for the future chapters
READ AT YOUR OWN RISK!!!! THIS IS ONE OF THE REASON I GOT HOOKED ON THE SERIES AFTER READING THE WEBNOVEL I SWEAR TO GOD THAT PERVERT HYAKUEN IS A FUCKING GENIUS.
Maro Easily got into top 4 without revealing all his cards including Renka. The top 4 got a consolation prize, each them get to pick a D-Rank card Maro and a girl choose the same card an Oni Girl Card with huge tits the girl is Anna the Daughter of the owner of Dungeon Mart you can actually see her with another contestants last chapter (the girl and the boy that sparkles ) Anna decided make a bet if they got matched they whoever wins gets the card if they don't get matched whoever advances gets the card if they both don't advance they will talk it out or something. To no ones surprise they got matched. Utamaro beats her since he used an item just before she could and that's enough for him to actually win. The other match was one sided the handsome guy beat his opponent convincingly the opponent actually becomes a relevant character in volume 6 lmaooooo before that he didn't appeared once! Finals is between the handsome guy and Maro I'll call the other guy Master since that's how Maro called him later on and I forgot 'their' name. Master is secretly an op. No kidding he works with the JSDF as a professional adventurer his siblings are one of the pioneer adventurers and the one tasked to clear B Class dungeons. In one of the dungeon runs his eldest bro died and he inherited hid cards with B RANK monsters in it. In battle he choose to use his own card which only has 1 C rank in it. During the battle even though they both use a D/D/C setup Maro realized that he's getting his ass beat, he notices that Master isn't even saying anything and yet he is commanding his monsters perfectly. Turns out the ops had a special technique that allows monsters and users to connect telepathically if they have enough connection. Maro having no choice he decided to copy it got his brain fried trying but still got to do it while bleeding from his nose eyes mouth and ear. Even though he learned link he still wasn't able to keep up Master's experience and strategy seeing his seeing his monsters get beat up relentlessly he is about to quit when a barely alive Renka says to him that the moment they accept the names given to them the monsters already fully trusted their master and are willing to die for them multiple times over. This snapped him back to reality and in desperation he give Renka the Amrita to heal her. Little did he know Amrita is the Key Item for Renka to MEGA EVOLVE! Literally. You see cards with Fallen Existence Already exist since the dungeon's first appearanc, but no one knows what it does except that it limits the card's power and thus card's with Fallen Existence skill are often considered garbage and defective. Turns out Fallen Existence allows for a monster to undergo a temporary Rank Up called Spiritual Recursion (webnovel TL) if they are given the key item this converts Fallen Existence into Spiritual Recursion. With Amrita Renka evolved from Zashiki Warashi into the Hindu Goddess Lakshmi's Japanese form Kisshouten. With the power boost Renka healed her teammates and destroyed Master's team in A few moves. This won Maro the tournament, after winning he became too popular as the first person to quite literally discover Mega Evolution in that world. As a result he became part of the top caste at school but soon he later found out that being part of the school caste and being popular are different, Maro's true intention in getting into the school caste is because of his crush but being popular doesn't mean his crush will reciprocate his feelings. With unnecessary attention at hand he decided to actually become a pro adventurer even though that's not what he wants in the first place. He just want to rizz some level 100 Gyatts end up with fanum tax(there is a chapter where he literally just talk about taxes with his father lmaoooooo)
I end up yapping too hard. Buy the novel and manga on AmazonJp or support Hyakuen on Kakuyomu by becoming a supporter and support Sagiyama-sensei by commissioning him on skeb.