He was literally played like a damn fiddle by people from another world. I thought that since he was working on retail store card, he must be experienced on negotiation or dealing with some people, so that he can gain advantage over his situation to survive in new environment. At the start of chapter 8, people around him are already planning to 'trick' him so that he can't return to Japan. He immediately lets his guard down when people from the kingdom treat him nicely, even though he himself said that he must be careful on another world. What an idiot.

Again, at the start of chapter 9, when he visited by little sister of an aristocrat, he suddenly very nervous to the point that his personality changes.

("Claire said that with a sweet smile, and I couldn’t help but think, “Please stop, or I might start falling for you... ”) .

Oh my God, what are you? A middle-schooler? For God's sake, you are an adult! Why are you nervous around people just because they are beautiful/good-looking?

Inside his backpack (and also his deck), there are numerous powerful cards that not yet / doesn't exist in another world. You know what happened next, right? Ding-ding-ding! You are correct! He easily giveaway those cards to people from the kingdom! What an outstanding move from our virg*n, genius, and promising Author! He doesn't have physical strength, experience on martial art or self-defense, and the only things that he can rely on is his deck, information, and knowledge about cards, yet he easily giving away some cards and crucial information, even his personal rare decks to the third party without consideration.