Why was Aqua responsible for humans from Earth? 70% of the planet's surface is covered in water, the dominant species is 60% water, the habitable atmosphere is thoroughly saturated with water vapor, and every single organism on it requires the liquid for survival.
It sounds ridiculous that Kazuma mistook a farm tractor with a truck and died from overwhelming pressure of being run over by it. But remember that he had stayed up for 3 days straight playing MMORPG. Staying up for that long results in hallucination.
It also puts undue stress on the heart. Together with the possible over consumption of energy drinks, it could account for his death by "shock".
The Anime also makes it a bit clearer... as, the sun was positioned such that he could only see the outline of the tractor.
Kazuma may seem like a hopeless otaku, but there's good reason to investigate the manga in the castle vault, even if that meant triggering the alarm. It has been said on numerous occasions that the locals of this world can't read Japanese, and the last time Kazuma saw manga was in the Crimson clan village, in the factory made by the same guy who designed/piloted "The Destroyer." This means that there's some Japanese transplant nearby targeting the royal family. After all, Iris's older brother got the divine artifact as a tribute from someone, and as of chapter 5 of volume 6, we know the artifact can be used to Kill and Replace people. Although he never learned of it, he was right. Lord Abelard, the "mayor" of Axel town, is not only the one who gave the divine artifact to Iris's brother, but he is also well aware of its powers and was planning to use it to Kill and Replace the crown prince so he could force Darkness to marry him.
Aqua's Hair Decoration (as seen here◊) consists in two small yellow spheres attached to a larger blue one. That's it, it's a simple three-dimensional model of a water molecule. Very fitting for a water goddess who's very in touch with modern society.
Why does Eris have levels in thief? Simple, she is the Goddess of Fortune and no other class relies on its Luck stat quite as much.
Thieves typically steal money. When Kazama registered, the Guild's clerk mentioned off-hand how a high Luck stat would make for a good merchant as well. The world's money is named after Eris.
Megumin's ideal man is someone who doesn't just loan money with an ulterior motive, has high goals, is a hard worker, and is an honest/serious person. Yunyun retorted that Megumin would attract the opposite; a lazy bum. While at first it looks like an ironic jab at Megumin falling for Kazuma, someone the complete opposite of what she looks for, but both are actually correct. While Kazuma's almost always a lazy bum, when the situation gets serious, he displays all the traits that Megumin looks for, which is why she fell in love with him.
Megumin's anime character intro PV claimed that she's the "Real Heroine", whereas Aqua's the "Main Heroine". While it seems like Megumin's being chuuni and trying to hog the spotlight, it's heavily implied that Konosuba is actually an in-universe novel series written by Arue about Megumin's adventures with Kazuma, making her claims valid.
Why do people think that Aqua is just a loony pretending to be the Goddess Aqua? Her character aside, it is not unusual for an Axis Cultist to act in that manner. Her adventurer registration card proclaims her as an Arch Priest of the Axis Cult after all, people figure she is just one with a physical resemblance to Aqua and a delusional sense of grandeur.
The doctrine of the Axis Cult is "Do as you will so long as it is not a crime", and their seventh doctrine is "You shall not endure. Drink when you want to drink. Eat when you want to eat. Because you might not get to eat it tomorrow ". Their teachings seem to revolve on instant gratification within legal (NOT moral) limits because of how ridiculously dangerous the setting is (any and ALL food, even the vegetables, are mobile and will fight back), but given the attitudes of some members, not even 'legal' really stops them. Aqua and female members of her Axis Cult do not seem to wear underwear. Try not to think why the little girl who tripped appears to be similarly attired.
Why did Sena focus so much on Kazuma? SHE DIDN'T! While she did jump to conclusions at first, the second he pointed out she was nuts...she backed off and became more friendly. Right up until her device caught out "I have never aided a member of the Demon King's army" as a lie...She's just got a case of Prosecutor's Fallacy, which is common in the Japanese justice system.
When Kazuma dies and first encounters Eris, his request is to be reborn into a family with "a pretty older step-sister and a pretty younger step-sister." You might think this is because he's a pervert, but it's more layered than that when you think about his party members Darkness and Megumin (who both fit the descriptions). It's very likely that he's asking that because he's grown attached to them, and as such, want people like them in his new life.
Doubles as heartwarming, as later Megumin and Darkness fall in love with him.
One might wonder why a holy crusader who wears yellow and white would be named "Darkness". As pointed out here, there is a status ailment called "Darkness" in the Final Fantasy games (games usually in a setting not unlike Konosuba.) Darkness severely affects the accuracy of a character's physical attacks, causing them to miss over and over. Now, think about our Darkness' own track record for hitting things with that sword.
When Aqua kept using Turn Undead on Verdia, why was it surprisingly effective? Turn Undead is a basic ability for priests in games like D&D, and is based on holy power. Aqua being a goddess, naturally has high holy power!
Additionally, Verdia's confusion as to Aqua's (and the party as a whole, in hindsight) power despite the fact that Axel is a low level starting area becomes awesome once you realize that the parties' levels are actually really low, it's just that the girl's Crippling Overspecialization makes them REALLY strong when utilized the right way:
Aqua, despite being a low level priest is actually a goddess, and thus has high holy power.
Megumin, despite being a low level arch wizard spends points in the strongest attack magic spell in the world, to the point of Explosion having nuke-like power.
Even Darkness is really tough and strong, even if she can't hit anything.
Additionally for Darkness, the only living beings in the world of Konosuba who have not been killed in one shot from a direct hit by Megumin's Explosion Magic have all been the Devil King's commanders...and Darkness. Even Vanir lost a life when being hit by it, with Darkness surviving the spell.
To top it all off, Kazuma, despite being the weakest class (adventurer), learns multiple utility spells and uses them in clever ways. Not to mention that he's the one who directs the girls to use their abilities for max effectiveness when it counts
In Verdia's defense, while he may have lost quite hilariously, what Verdia went through to be defeated deserves mention as well. He took a Turn Undead, a Sacred Turn Undead, Megumin's Explosion magic, Sacred Create Water, Kazuma stealing his head and kicking it around, and finally a second Sacred Turn Undead. Not to mention that the Turn Undeads, Sacred Turn Undeads, and Sacred Create Water came from Aqua, a goddess.
Megumin, upon explaining her love of explosion magic, says she puts points into her magic's power and reduced casting time as she levels. Not counting montages, she takes less time to cast Explosion throughout the series.
The Steal skill is said to steal more valuable items from the target based on the level of the skill and the user's Luck Stat. So why does Kazuma, who is stated to have unusually high luck, keep stealing panties, which only have modest value? Because the spell determines "value" based on the thoughts and feelings of the user, and Kazuma is an irrepressible pervert.
As Kazuma posed while beginning to play rock-paper-scissors, he formed a spell-circle from his hands. While this was probably purely Rule of Cool and Rule of Funny, Aqua insultingly stated that it was his special ability. The spell-circle gives credence to her claims. He's never lost since he was a kid, after all. Even Kazuma quizzacly looks at his hand, as if even his own luck was a mystery to him.
In volume 9, Kazuma tells Megumin that he was a huge jerk to his parents, who paid for him to go to an expensive high school, and repaid their kindness by becoming a shut-in and never actually showed up to class. This may have had some sway on their opinion on him.
As wonderfully karmic as Alderp's fate is in Volume 7, we all have to remember he has a son, Walther, who is actually a decent human being. Literally overnight, Walther's life was completely turned upside down by his father's bad behavior. Vanir makes a point of saying Alderp's family fortune is gone. This means Walther's both in immediate, horrific poverty, and under suspicion from the entire kingdom because they don't know about "Max," and he's the first place Alderp might go to get aid. One can only hope the poor guy has some kind of other career he can get into than being the son of nobility, or he's SOL.
Luckily, Volume 8 Chapter 1 mentions that the authorities recognized his innocence and that he's helping Darkness's father run Axel.
There's a bit surrounding Kazuma's unfortunate encounter with the Winter Shogun. When Kazuma revives, there's a huge pool of blood in the distance, implying that either: Aqua revived Kazuma in a completely new body and his decapitated one is still there, or somebody had to carry and hold his severed head in place while Aqua did her thing. Either way, such a sight can't be good for the girls.
It might have been Played for Laughs, but Wiz was basically on the verge of passing away for the last 4 episodes of season 2.
The main characters behave the way they do because Konosuba is a parody. But if you think about it, all of them are extremely unpleasant people. If anything, infantile and narcissistic as she may be, Aqua is the least pathological of the lot.