The first piece of information you find, following up on the name Ikareimaru, dates back far longer than one might first expect, drawing up old history from 1877. The Satsuma Rebellion--Seinan Sensō (Southwestern War)--was a revolt of disaffected samurai against the new imperial government, nine years into the Meiji Era. Its name comes from Satsuma Domain or Satsuma-han, also known as Kagoshima Domain, and was established during the Edo period. It is associated with the provinces of Satsuma, Osumi and Hyūga in modern-day Kagoshima prefecture and Miyazaki prefecture on the island of Kyūshū, Japan. Satsuma had been influential in the Restoration and become home to many unemployed samurai after military reforms rendered their status obsolete.
The Rebellion lasted from January 29, 1877 until September of that year, when it was decisively crushed and its leader, Saigō Takamori, ended his life.
Saigō's rebellion was the last and most serious of a series of armed uprisings against the new government.
Saigō initially disagreed with the modernization of Japan and the opening of commerce with the West. He famously opposed the construction of a railway network, insisting that money should rather be spent on military modernization.
Saigō did insist, however, that Japan should go to war with Korea in the Seikanron debate of 1873 due to Korea's refusal to recognize the legitimacy of the Emperor Meiji as head of state of the Empire of Japan. Other Japanese leaders strongly opposed these plans, partly from budgetary considerations, and partly from realization of the weakness of Japan compared with the western countries from what they had witnessed during the Iwakura Mission. Saigō resigned from all of his government positions in protest and returned to his hometown of Kagoshima.
Shortly thereafter, a private military academy was established in Kagoshima for the faithful samurai who had also resigned their posts in order to follow him from Tokyo. These disaffected samurai came to dominate the Kagoshima government, and fearing a rebellion, the government sent warships to Kagoshima to remove weapons from the Kagoshima arsenal.
Ironically, this provoked open conflict, although with the elimination of samurai rice stipends in 1877, tensions were already extremely high. Although greatly dismayed by the revolt, Saigō was reluctantly persuaded to lead the rebels against the central government.
On his return to Kobe on February 12, Hayashi met with General Yamagata Aritomo and Ito Hirobumi, and it was decided that the Imperial Japanese Army would need to be sent to Kagoshima to prevent the revolt from spreading to other areas of the country sympathetic to Saigō.
On the same day, Saigō met with his lieutenants Kirino Toshiaki and Shinohara Kunimoto and announced his intention of marching to Tokyo to ask questions of the government. Rejecting large numbers of volunteers, he made no attempt to contact any of the other domains for support, and no troops were left at Kagoshima to secure his base against an attack. To aid in the air of legality, Saigō wore his army uniform.
Marching north, his army was hampered by the deepest snow
fall Satsuma had seen in more than 50 years. This was the omen that is often looked to historically for the omen of Saigo's death, myth telling that it was also the doing of the man who saw to his mortal wounds in battle.
The rebellion was suppressed in a few months by the central government's army, a huge mixed force of 300,000 samurai officers and conscript soldiers under Kawamura Sumiyoshi. The Satsum
a rebels numbered around 40,000, dwindling to about 400 at the final stand at the Battle of Shiroyama. Although they fought for the preservation of the role of the samurai, they used Western military methods, guns and cannons; all contemporary depictions of Saigō Takamori depict him garbed in Western-style uniform. At the end of the conflict, running out of material and ammunition, they had to fall back to close-quarter tactics and the use of swords, bows and arrows. Saigō and his remaining samurai were pushed back to Kagoshima where, in a final battle, the Battle of Shiroyama, Imperial Army troops under the command of General Yamagata Aritomo and marines under the command of Admiral Kawamura Sumiyoshi outnumbered Saigō 60-to-1.
During the battle, Saigō was badly injured in the hip. However, the exact manner of his death is unknown.
The accounts of his subordinates claim either that he uprighted himself and committed seppuku after his injury, or that he requested that the comrade Beppu Shinsuke assist his suicide. In debate, some scholars have suggested that neither is the case, and that Saigō may have gone into shock following his wound, losing his ability to speak. Several comrades upon seeing him in this state, would have severed his head, assisting him in the warrior's suicide they knew he would have wished. Later, they would have said that he committed seppuku in order to preserve his status as a true samurai.
Regardless, there are also accounts by Western forces within the battle that, rather than going into shock for his loss of speech and mobility, the one resonsible for his injury also saw fit to freeze his blood and flesh, before killing him with a 'blade of ice' in the manner befitting a samurai, effectively offering an assisted seppuku himself. Whatever story holds true, all accounts tell of Saigo's defeat at the hand of the mysterious warrior known only to his comrades as Ikareimaru.
To this day it is unknown which side the man fought for, or what his intentions were aside from apparently quelling the rebellion. No documentation of him appears in conscription for either side, and many accounts claim he simply appeared in the fray, a stranger to both sides though fighting every man equally to cut a swath toward Saigo.
Myth has it Ikareimaru was the snowstorm personified, an angered yokai or the slumbering spirit of Shiroyama that was woken by the warring, and annoyed by the disturbence, punished the one he saw fit to blame.
Nevertheless, when the tide of battle turned and eyes were cast to look for the warrior to dispatch the last samurai, no trace of the mysterious Ikareimaru could be found, not even footprints in the snow.
The Kanji lettering used to spell the name Ikareimaru, when taken individually for symbollic meanings, piece together an interesting idiom; 'Regards (regarding); (always) as ever; (beauty) [the] beautiful; (chill) cold'.
The suffix 'maru' is a masculine to relate these meaning back to the subject, and when taken literally, imply 'The boy who--' to this lyrical moniker.