Russia and China are a part of the Allied Forces during WWI and are occasionally interacting with each other during the main timeline, such as when they both are on a plane and Russia tells China not to worry about giving him a parachute as the snow would cushion his fall, claiming that he's Russia and he knows everything about snow.
Russia also appears to be the one that comforts China after the latter tells him about his bad state of relationship with Japan while drinking at a bar, assuring him that he and Japan will get along in the future before giving his trademark creepy smile and adding mercilessly that everyone would soon become one with Russia no matter what the situation upholds.
After China lost the Opium Wars and is forced to do England and France's bidding, he returns home tiredly to find Russia eating an early dinner at his house. He is shocked and demands an answer from him, Russia claims that he's just eating dinner.
In one character relationship note, Russia is shown to have a sort of unexplained desire towards China.
Before the onset of the Boxer Rebellion, Russia tells England and France to stop taking more of China's territories as he is clearly upset of their demands. China looks relieved for a second but when Russia states that China wants to live with him, he immediately refutes with a big NO.
In modern times, China is complaining to his panda about how many countries are bullying him right now and when he says that maybe Russia is planning to backstab him with a suspicious tone, the panda removes its head, revealing a surprised Russia who soon asks a screaming China how does he find out.
RoChu is a popular pairing in the Hetalia series. Their relationship isn't dug further in the series but in real life, they both have close historical and political ties that date back to the 1600s. One of the most famous timelines to state is the Cold War when they established a treaty of friendship, making them the closest allies against most of the world during that period.
The Sino-Soviet split in the 1960s is often portrayed as a bad break-up, as their conditions were bad after the split. They eventually grew and had their countries improved in terms of trades and their diplomatic ties were restored, making Russia and China to appear as close allies even in modern times.