Comic Trio: More often than they'd like. France at one point even compares them to the Three Stooges.
One Head Taller: Germany and Italy are often portrayed as this, even though Italy is not exactly one head shorter than Germany, but this usually happens to accentuate the fact that they're supposed to be a couple. In fact, the height difference is more obvious in Germany and Japan's case.
Only Sane Man: Germany. Japan is this occasionally to Germany and Italy, especially in anime artwork. And, in one strip, Italy plays the role of the sensible one to Japan and Germany's new dieting habits.
Ship Tease: Germany/Italy is one of the most heavily hinted at pairings in the series, second only to Hungary/Austria in implied official-ness. However, the series has no confirmed Official Couples yet.
Defeat Means Friendship: Despite Italy kicking his ass as a child, Turkey and him are on good terms in present in present day. Turkey even fondly recalling the incident when Italy and Germany come to visit his house and remarking that Italy had really mellowed out since the old days. Germany was shocked to say the least...
He's usually drawn with eyes that resemble equals signs (though in the first episode, he had his eyes open before he closed them and in episode 15, his eyes flew open when he found out that Japan had pornographic pictures in one of his books).
Lampshaded in episode 41, where Italy opens his eyes as he wakes up.
I tried opening my eyes for a change. ~Italy Veneziano
"You can order me around and I'll disappoint you!"
Hidden Depths: Later strips and blog posts have pointed towards him having these. Italy apparently enjoys gory horror movies that squick out even Germany, once kicked the Ottoman Empire's ass as a kid, presumably with just a longbow and his own physical strength and skills, and he's a gifted fencer, artist, cook, and skilled at trade.
Idiot Hair: Italy has one single hair curl on his left side, known to be an erogenous zone when touched. A couple incidents of Germany doing such was an Accidental Pervert situation.
Informed Ability: in a recent blog post, Himayura said Italy's fencing abilities are unusually strong. We have never seen Italy so much as hold a sword of any kind in canon let alone fight using one. But then again, would you really expect him to?
Innocent Fanservice Guy: Italy is rather eager to take his clothes off when he gets ready to nap or sleep. This often gets him bitched out (by Japan) or kicked away, as Austria showed him once.
And how! Even the title wants you to know this. It's actually even Truth in Television to an extent - historians say that Germany might have actually been able to defeat Russia in WWII had they not been so busy bailing Italy out of Greece, pushing the Russian campaign dangerously close to winter.
"Hetalia" (Hetare = useless or wimpy + Italia) is actually a well-known Japanese nickname of Italy among WWII historians and enthusiasts.
Ocular Gushers: Italy's tears are pressurized like a water hose, sometimes getting all over other people.
Picky Eater: He doesn't hide his dislike of Germany's sausages (even though he once sang to Germany that sausages and cheese from Germany tasted good... or at least better than English food, anyway). He's been this way ever since he was a child, as Chibitalia, he once spurred food that Holy Rome left for him because it wasn't good enough.
Trouser Space: In one comic, Italy pulls a pair of boxers out of his own pair.
Used to Be a Sweet Kid: Inverted. He's a very sweet adult, and was a sweet kid under Austria's care, and Rome's as well, but according to Turkey, he used to be a pretty wild kid during the period of time between Rome's fall and being conqueror-ed by Austria.
Icy Blue Eyes: He has pale blue eyes and a stoic personality to match.
Laser-Guided Amnesia: It has been hinted that he really is the missing Holy Roman Empire, only grown up and without any recollection of his childhood. That comes across in the Valentine's strip, which ends with Germany possibly starting to recover his memories.
My Country, Right or Wrong: Well...yeah, but WWII Germany in particular fits this trope. He's seen reacting with open shock and disgust at some of the orders he's given (like taking Austria away by force in the "Anschluss", despite Hungary and Italy's pleas), yet he can't do anything but obey.
Neat Freak: One strip has Italy cooking and Germany cleaning up his mess right next to him.
Only One Name: Unlike the other characters, only his first name was ever given. According to Word of God, we allegedly will find out why...someday. Fans generally give him the same last name as his brother Prussia if one is required.
Super Strength: Germany's grip is strong enough to break a metal spoon in half.
Trademark Favorite Food: If you ask South Italy or France, he's a "potato freak", but he doesn't eat them often. When France spots him undercover, it's the way he's eating that gives him away. He also has wursts and beer.
Come on, this is Japan we're talking about, how can he not be one? It's shown a little in the strips as well-like praising 2D over 3D, and casually having confidence-stealing erotic woodblock prints in his art book collection.
In The Stinger at the end of one of the anime episodes, Italy is looking through Japan's architecture and art books and comments how Japanese art is very nice — until he happens upon a picture of erotic Japanese art and sighs, "It's porn."
Cloud Cuckoo Lander: Not as obvious as the rest of the cast, but he definitely has his moments.
Cryptic Conversation: The art of being as vague as possible ("We'll exercise discretion", "I'll see what I can do", "Perhaps next time", etc.) without actually saying "no" is a specialty of Japan's.
Cute Bruiser: Official art often shows him wielding or carrying around a katana, and during the Christmas 2010 event he puts up a hell of a fight against the mysterious kidnapping intruder with a rolled-up poster.
Fish out of Water: He feels a bit strange sometimes when interacting with the other Axis members.
Flower Motifs: His human first name, Kiku, means "chrysanthemum", a flower with great symbolic importance in his country. Cherry Blossoms are also often associated with him, especially after his introduction in the anime among a shower of them; Japanese fans have even adopted "Sakura" as a Fan Nickname for his Gender Flipped self.
Hates Being Touched: It took Italy 3 seconds to get Japan very angry just by hugging him. (compare to Greece's intentional attempt to get Japan mad: only a single puff of anger after a solid minute of effort)
Hikikomori: Japan spent 200 years as one before America arrived at his doorstep with Commodore Perry. More specifically, Japan identifies himself as a hikikomori when he sees the American boats and almost has a Freak Out, and also when he does a Sunako in response to Netherlands opening the door to his room ("Don't shed sunlight on me! I'm melting!").
Inscrutable Oriental: It isn't too often you see a Japanese character in a Japanese series characterized this way. Made even more noticeable by all of Japan's other Far Eastern siblings not fitting this trope in the slightest (with the possible exception of Hong Kong).
Germany: ...Italy is the only friend I've got... and I can never tell what Japan is thinking..."
China: You can call me China! (...) So, would you mind telling me your name?
Child!Japan: (bows) Konnichiwa. It's nice to meet you China, where the sun sets. My name is Japan, I am from where the sun rises.
China: Oh! Well, that wasn't very nice! note In Asian cultures, places where the sun rises are considered good and places where it sets are considered bad, so Japan basically said: "Hello, stupid China, I'm Japan and I'm way better than you.
Truth in Television, as this is practically how Japan addressed itself in their first diplomatic document to China (The Emperor of the Land of the Rising Sun to the Emperor of the Land of the Setting Sun...). The Chinese Emperor was of course pissed off, saying "bring not those impolite states before me!"
Kick the Dog: His stabbing of China in the back, presumably meant to symbolize the first Sino-Japanese war, which leaves a permanent scar on China's back. Though some fans seem to like emphasizing this single action of Japan a little too much, conveniently forgetting that there's more to him than that. YMMV, but by the standards of a Japanese work, this is actually astoundingly honest about some of the more regrettable actions of the Japanese Empire.
Kimono Is Traditional: Often wears a yukata or kimono when he's at home (although he does sometimes wear more Western clothing), and even more often gets turned into Kimono Fanservice in fanworks.
Mighty Whitey and Mellow Yellow: Okay, he isn't female (except in the occasional Gender Flip fanwork), but his personality is the one most closely tied to Asian stereotypes and the most common partner for him in the Western fandom is Greece whose canon interest in his culture and language is often brought up in them, with in the Japanese fandom, it's England who stayed over at his house to learn more about his culture during their alliance. It's interesting to note that on Pixiv, England x Japan far outstrips any other ships, at 30k+. Keep in mind that Japan is the fandom bicycle...
In spite of Japan easily being one of the most stoic members of the cast, he's proven capable of losing his cool on quite a few occasions. Especially over physical intimacy, food prices, 2D, and Russia.
After doing some touristy things with Italy, he was shown smiling like an idiot and exclaiming "PASTA!" Italians are apparently contagious.
Yamato Nadeshiko: Never mind his gender, Japan by himself is the character in the series who's the closest to being a Yamato Nadeshiko, especially in the modern day strips. He's polite, soft-spoken, wears traditional Japanese clothing outside of meetings, and while he has his share of bizarrequirks like every other character in Hetalia does, they tend to be subtler. This has led to some people accusing him of being a Flat Character, even though these traits are frequently used for humor/parody (e.g. he's often too polite for his own good or doesn't let his politeness get in the way of him being passive-aggressive).