More things may be added later on! If you want to know more, I suggest this wonderful blog: [link]
Do not butcher a language, this means using things like "Fratello", "Bruder", "Mon cheri", etc. DO NOT USE THOSE UNLESS THE CHARACTER IS TALKING TO ANOTHER CHARACTER WHOM SPEAKS THAT LANGUAGE FLUENTLY.
Do not make a character call their sibling only by brother, sister, etc. Saying those in different languages are not an exception! It's more likely for siblings to call each other by their names, as I call my big sister Crystal. An exception for using brother/sister/etc is when the character is talking to someone whom does not know their sibling or doesn't talk to them very often.
Belarus is a good acrobat and can see ghosts, as confirmed by Hetalia WikiNet.
Germany and Prussia are both EXTREMELY UNCOMFORTABLE WHEN IT COMES TO SEXUAL CONTACT, ROMANTIC RELATIONSHIPS, AND PLATONIC RELATIONSHIPS. For that point, none of them are a beautiful German sexgod, sorry to burst your bubble!
(I cannot bother to find any strips of Prussia, however, here are some hilarious strips of Germany.)
Prussia is very awkward and even anti-social, for christ sakes, he even has his own diary and runs a blog! So do not make him someone whom is always like "I AM AWESOME, AND YOU ARE NOT!", or make him extremely infatuated with sex.
ENGLAND DOES NOT HATE ANY KIND OF FOOD FROM THE USA. He was actually confirmed to love McDonalds!
America is very insecure for having no history of his own before he became a colony.
Canada is a very good tactician!
England has a tattoo of an electric guitar somewhere on his body, I like to think that it's on his back.
CHINA HAS A PANDA SHAPED BIRTHMARK ON HIS BUTT.
France tends to be very grumpy when awoken, and he also may be a sleepwalker.
Russia's metal pipe is actually kinda like Mary Poppin's bag, he can put things in it! The majority of time it is food, however.
China loves online gaming!
Germany loves Maid Cafes, as quoted in his diary, he loves the uniforms they wear.
France also said once in the comic that Germany tends to run around naked more than Italy does.
Italy suffers from a condition called Phimosis, so I'd suggest doing some research on that before writing a smut fic with him. (WARNING, THAT ARTICLE DOES INDEED HAVE PENIS IN IT.)
Japan really, really hates cockroaches, so much that sometimes he'll even cry over them.
Prussia hates the word 'mark' for whatever reason, though, I have a feeling it is because of Marxism, which is known for being in Russia, and Prussia does indeed hate Russia.
Austria apparently spent some time in a wheelchair, as said in a character note.
The flowers that Hungary wears in her hair represent Lake Balton.
Switzerland was once hired by the Vatican as a mercenary. It seems to be his dream job.
THE ITALY BROTHERS WOULD NOT LIVE TOGETHER. [link]
DO NOT MAKE FICS BASED OFF OF TRAGIC EVENTS IN THAT COUNTRY, THIS INCLUDES 9/11, THE JAPANESE TSUNAMI, ETC. IT CAN BE INTERPRETED AS OFFENSIVE UNLESS YOU DO NUMEROUS AMOUNTS OF RESEARCH.
CANADA, LATVIA, ETC, DO NOT HAVE A STUTTER, HOWEVER IF YOU DO GIVE THEM A STUTTER, PLEASE EDUCATE YOURSELF ON IT.
Now here's something that pisses me off to no end, and it doesn't just go for fic writers, this goes for RPers as well: DO NOT TYPE OUT ACCENTS, EVER. DO NOT TYPE RIKE ZHIS, ETC. IT'S PLAIN ANNOYING, SOMETIMES HARD TO READ, AND EVEN OFFENSIVE SOMETIMES.
Fem!Italy is in no way like normal Italy. She is more hotheaded and strong. Here are some examples.
Also, taken from the Hetawiki:
Please, educate yourself on gay sex before writing it.
Romano is not always angry, in the manga he can be pretty calm. He can call Spain a shithead, asshole or whatever he's insulted him with, but sometimes he's actually chill with Spain.
SPAIN HAS ACTUALLY GOTTEN ANGRY AT ROMANO NUMEROUS TIMES IN THE MANGA.
Himaruya has confirmed that Spain and Finland have a few mental "cracks", and could possibly be even worse than Russia.
Here's a nice website for simple everyday foreign phrases.
Spain would actually a very strict guardian, Austria would be more carefree.
DO NOT USE THINGS LIKE DOITSU, IGGY, ONII-CHAN, 'DA', ETC IN YOUR FIC. This also goes for excessive uses of aru, da ze, ve, etc.
SOME THINGS YOU SHOULD READ IF YOU'RE GOING TO MAKE KOREA SPEAK IN KOREAN.
Tips for writing Japan.
BELARUS IS NOT SOME KIND OF FREAKISH STALKER, RAPIST, ETC. NEITHER DOES SHE ALWAYS WANT TO MARRY RUSSIA.
France confirmed that HRE died in the manga.
I cannot stress this enough:
NOBODY IN HETALIA IS A RAPIST, PEDOPHILE, PSYCHO, ETC. DO NOT EVEN IMPLY ANY OF THIS, RAPE NOR PEDOPHILIA IS FUNNY.
Germany has three dogs, and their names are Blackie, Berlitz, and Aster.
Apparently, Japan sees the world as a RPG, as shown in a strip.
Japan's blood type would be A, according to Himaruya.
As stated by Himaruya, England's eyebrows are actually trimmed, but they do not appear that way. He also said that Ireland has thick brows as well, but with a "different feel" to them.
France is a member of the Academy Gourmet Food Club along with Turkey and China.
After a heart pops out of Russia's chest, he claims it as his own, saying "it just pops out of me sometimes". Some fans see it as possibly symbolic of the heartbreak he's suffered in his time, or a representation of just how mentally damaged he is. (taken from the Hetalia WikiNet)
China has a scar on his back from when Japan attacked him.
A post put up by Himaruya not long ago says that Australia's human name could be Ralph, Christian, Kyle, and/or Jett.
Austria seems to be on bad levels with Australia, as he finds it frustrating that their nations names are similar in many languages.
In a character note, it says that Austria only wears glasses not because he has bad eyesight, but because he thinks he would look plain without them.
Belgium's possible human names could be Laura, Emma, Anri, and Manon, as said in a post by Himaruya.
Denmark's possible human names could be Andersen, Christensen, Arnesen, Simon Densen, Abel, Mikkel, Magnus, and Bertram. "Densen" is possibly his surname.
Estonia can play the piano!
Hong Kong's possible human names could be Wang Jia Long (Wong Kha Loung in Cantonese) and Li Xiao Chun (Lei Siu Chun in Cantonese). He was also given the English name Leon.
Hutt River owns a Bilby, a small marsupial native to Australia.
Iceland's possible human names could be Emil Steilsson, Sigurður, Eiríkur,"Erik" and Egill.
On his blog, Himaruya mentions that Pakistan was likely in the background somewhere during the Halloween event thinking "That bastard..." when spotting India. He goes on to say that the history between those two countries is full of great interactions. (Taken from Hetalia WikiNet)
Korea's habit of claiming things originated from him most likely comes from the taegeuk on his flag. The taegeuk refers to the ultimate reality from which all things and values originate according to oriental philosophy. (Taken from Hetalia WikiNet.)
When a fan asked how Ladonia came to receive his scar, Himaruya responded that the scar is actually paint.
During the Christmas 2011 event, he stated that his favourite Pokémon is Stunfisk.
The "desktop mascot" of Latvia reveals that he has an apparent dislike for things such as the Livonian Order, Alexander Lukashenko, the Teutonic Order, and Cossacks.
Liechtenstein's possible human names could be Erica/Erika, Sisia, Elise, and Eva. Her possible surname is Vogel.
Molossia owns a large dog whose bark is strange. The other nations are are initially unsure of what it is. (Taken from the Hetalia WikiNet)
Molossia's mobster-like appearance (his white "zoot suit", sunglasses, and slick hair) is likely a reference to the history of organized crime in the Nevadan city of Las Vegas (and to a lesser extent, Reno) and the stereotypes associated with it. Gangsters, including Bugsy Siegel and members of the Al Capone gang, ran casinos there in the early to mid 20th century. (Taken from the Hetalia WikiNet)
Netherlands is xenophobic.
Netherlands' possible human names are Tim, Govert, Abel, and Mogens.
Norway's curl represents the Fjords.
Norway's possible human names are Lukas Bondevik, Børre Thomassen, Bjørn, Kjetil, Knut, and Sigurd. Bondevik and Thomassen seem to be his potential surnames.
Sealand has a fondness for chocolate bars, and likes to browse the internet and play games as a hobby, as revealed by the "desktop mascots" from the official blog. (Taken from the Hetalia WikiNet)
According to a note by Hidekaz Himaruya, Spain was only originally going to speak in the Kansai dialect whenever he was excited or around people that were close to him. In addition, Himaruya also revealed that Spain might as well have "two faces" like Russia. He also stated in another note that while Spain treated Romano kindly, he was stern to his other colonies. Himaruya himself wondered why Spain was "historically stuck on Italy" that much, as it was stuck spending so much money to protect it and struggled to try to get it back after losing it in the War of the Spanish Succession. (Taken from Hetalia WikiNet)
Switzerland has three pet goats: Eiger, Jungfrau, and Mönch, named after Switzerland's famous mountains.
Taiwan's possible human names could be Lin Yi Ling or Xiao Mei.
Thailand has a verbal tic of saying "ana (อะนะ)".
Thailand has a pet elephant named Toto!
Ukraine's possible human names could be Irunya Chernenko, Maria, and Sofia.
Greece dislikes Albania, as said in Greece's bio and relationship chart.
Luxembourg has a small dog named Peltze.
New Zealand has a pet kiwi (the bird, not the fruit.)
New Zealand is androgynous.
Romano is actually flirtatious towards girls, and is more cold/aloof towards men.