Russia/China

Russia/China is a extremely common pairing involving the characters Russia and China. The pairing is sometimes referred to as RoChu, a portmanteau based off the Japanese pronunciations of the two countries' names, RusChi, a portmanteau based off the English pronunciations of their names, Hóngsè zǔ (红色组) "Red Pairing", due to their flags both being red, and Communist Duo, because both have a history of communism.

The War Years
In the main storyline, Russia and China were both members of the Allied Forces and they were occasionally shown interacting with one another, such as China being in the same airplane with Russia when the latter claimed that he didn't need a parachute as the snow would cushion his fall.

Russia was also the one who comforted China after the latter told him about the bad state of his relationship with Japan while drinking at a bar, reassuring him that he and Japan would definitely get along better in the future before smiling creepily and adding that everyone would soon become one with Russia anyway.

In time of war and also in time of peace Russia and China always supported each other as countries. In WWII Russia won one of China's territories but instead of keeping it he gave it back to China.

Other Strips
After China lost the Opium Wars and was forced to do England and France's bidding, he returned home only to find Russia eating an early dinner at his house.

Before the onset of the Boxer Rebellion, Russia told England and France to stop asking China for more of his territories because their demands were clearly upsetting him. He then went on to state that China wanted to live with him anyway, a statement that China refused with a big NO.

In a strip set during an unspecified period of time (but most likely in modern times), China hugs his panda while lamenting about how many other countries are bullying him now. After he declares to the panda that he suspects Russia of planning to stab him in the back, the panda removes its head, revealing a surprised Russia who asks China how he found out.

When one member was absent from the G8 meeting and the members present were trying to remember who the missing country was, Russia suggested twice that it had to be China (it turned out to be Canada - China isn't even a member of the G8). This was compounded in the anime adaptation by England accusing Russia of only saying this because he wanted China to be at the meeting.

Also some fans interpreted that in Comic Diary 7 that the person inside the costume is Russia because of the yandere face seen in the last panel.

Fan Speculations
Even though Russia and China's relationship has been touched upon only a couple of times in the webcomic so far, the two countries in real life have had a long history of relations with each other, extending as far back as the 1600s, and a popular concept in Russia/China fanworks is that Russia first met China as a child and that they grew to know each other well before Russia became who we see today. Thus, it's presumed that China would have seen more of Russia than the creepy psychopath the other countries see him as. This could also be brought back to the motherly aspect of China's personality, seeing Russia as a child in an adult body and feeling the urge to take care of him as he had done with his younger siblings, perhaps as a way of filling the gap after they left him to become independent.

Many Russia/China fanworks also focus on their relationship during the Cold War when the two countries established a treaty of friendship that made them each other's closest allies against most of the rest of the world. The Sino-Soviet split in the 1960s is often portrayed by fans as a bad break-up between the two characters in this manner, with them eventually having a reconciliation of sorts in modern times where diplomatic and trade relations between the two countries have improved once again.

Others present their relationship as a fairly equal one or even one in which China has the upper hand over Russia with the reasoning that China has enough experience and wisdom accumulated over 4000 years to know how to deal with Russia's childishly sadistic behavior and never does become a victim of Russia in the way the countries who become part of the Soviet Union do. Still others portray their relationship as one of codependence or mere convenience, with China being dependent on Russia for monetary help and Russia dependent on China to keep him at least partially sane.

A striking parallelism is also present, as Russia has siblings and subordinates that form a large family (the Baltic nations, Belarus and Ukraine) while China has his East Asian family.

Webcomic: Main Storyline

 * Hetalia Chapter 2: Allied Forces

Webcomic: Extra Stories

 * The Story of Little China and Teensy Japan
 * Italy's Big Brother
 * China Melancholy
 * Before the Boxer Rebellion