Tsesarevich (Tsarevich) Alexei Nikolaevich of Russia (In Russian Царевич Алексей Николаевич) (August 12, 1904 - July 17, 1918), of the House of Romanov, was a Tsarevich of Russia and was the youngest child of Tsar Nicholas II of Russia and Alexandra of Hesse.
His older siblings included sisters Olga, Tatiana, Maria and Anastasia. Alexei was reportedly closest to Anastasia out of all of his sisters.
He had a medical condition called hemophilia from his mother Alexandra and traced back to her own maternal grandmother Queen Victoria. However, recent scholarship has called into question the nature and severity of his ailment.
He was assassinated along with the rest of his family in 1918. His body was missing, along with a Grand Duchess (Tatiana, Maria or Anastasia) when the bodies were found.
In 2001, he and his family were canonized by the Russian Orthodox Church.
The significance of Alexei is threefold. Firstly, Alexei was the heir to the throne despite being the fifth and last child of Nicholas II and Alexandra. Women had been barred from the succession by Paul I (1754-1801, ruled 1796-1801), in revenge upon his mother, Catherine II ('the Great'). Alexei was named after Aleksey I of Russia, who ruled from 1645 to 1676, known as 'the quiet' and father of Peter the Great.
In the first draft of Tsar Nicholas II's 1917 abdication, the intention was that the 12-year old Alexei would ascend to the throne under a regency. However, due to Nicholas' wish that Alexei should not be separated from the family, and in view of his crippling illness, the final draft included the abdication both of father and son in favour of Nicholas' younger brother Michael II.
Secondly, his hemophilia was integral to the rise of Grigori Rasputin. One of the many things Rasputin did that unintentionally weakened the fall of the Romanov's was he told the Czar that the war would only be won once he (Czar Nicholas II) took command of the Russian Army. This was a serious mistake when the Czar did follow his advice, because he had absolutely no military experience. The tsaritsa, Empress Alexandra, a deeply religious woman, came to rely upon Grigori Rasputin and believe in his ability to help Alexei where conventional doctors failed. This theme is explored in Robert K. Massie's peerless "Nicholas & Alexandra". It is not too far-fetched to see that if Alexei had not suffered so terribly, Rasputin could never have gained such influence over Russian politics during the First World War, which at the very least hastened the collapse of Romanov rule.
Lastly, Alexei being hemophiliac seriously diverted the attention of his father, Nicholas II, and the rest of the Romanov's from the war and the government to caring for Alexei. If Alexei had not been hemophiliac, Nicholas II might have had the time to personally look after the happenings in Petrograd which started off the Russian Revolution.