Uguisumaru was made by a swordsmith named Bizen Tomonari, of the Kobizen clan. At that time, he has been said to be one of the three most famous smiths (alongside Munechika and Oohara Yasutsuna (of Doujigiri Yasutsuna). Uguisumaru has been one of his earliest works, making him one of the oldest swords ingame so far.
Just like Mikazuki Munechika, Uguisumaru was never meant to be used in an actual battle and has in fact never been used to kill people. Instead, he has always been treated as a work of art.
This sword had first been in possesion of the Ashikaga shogun family, until the 6th shogun Ashikaga Yoshinori awarded him to the Ogasawara family for their achievements in the Yuuki Kassen (a civil war between Muromachi Bakufu and Yuuki Clan) during the year 1439. He then became the family sword of Ogasawara and was handed down generation by generation.
After the Meiji restoration, he went into the possession of the Baron Sou family. (There's been a rumour going on about that Uguisumaru was sold, because the Ogasawara family was too poor.) Afterwards he got bought by Viscount Akimoto.
Later he was bought by Tanaka Mitsuaki, a politician during the Meiji era.
Tanaka Mitsuaki presented Uguisumaru to the Meiji emperor in 1907, when the Emperor was in Ibaraki for the special military presentation.
Before he was presented to the Emperor, his body is full of blisters (膨れ) due to air trapped inside the metal. He almost became useless both as a sword and as a work of art. So they had to fix him. Some have speculated this as the reason he wears Western style, but another speculation is that when he was presented to the Emperor it was during a military presentation, so his uniform may be of a soldier’s.
This was Tomonari's fault, because he didn't fold the metal tight enough during the forge.
His phrases “treasure your life!” and “don’t mind what other people say” may come from this depressing part of his past. He wasn’t born perfect and neither did people take good care of him, which leads him to treasure what little he’s got.
They might also hint at the fact that a lot of his masters died of illness at a young age.
His rather easy-going and relaxed personality might be because of his notable age and because he was treated as a piece of art rather than a sword to take other people's lives, which is why he shows interest in battles, but without the intent to kill.
He says he doesn’t know where his name comes from. He had been called Uguisumaru ever since the Muromachi era. But story says that when he was drawn from his sheath, the singing sound of the uguisu can be heard even when it’s not springtime.
His liking for tea may be related to the Bizen no Kuni being a famous producer of teaware.
The strange little jacket he wears on his arms are gauntlets called Kote (篭手).
His color image is based on the japanese bush warbler.
Uguisumaru is the 'Tea' part of R4's nickname: Ice Royal Milk Tea. Sometimes he is also referred as the King of said quartet.