Date Masamune is one of the most famous feudal era daimyo of Japan. Nicknamed by later generations as Dokuganryuu (one-eyed dragon) due to him having only one eye, and known for his helmet adorned with a crescent moon, he's most famous for (almost) single-handedly unifying Northern Japan with great ferocity and possessing good insight about the future, especially in regards to foreign trade, Christianity and allegiances. He was an ally of Ieyasu during the Battle of Sekigahara and later during the Siege of Osaka.

He married at thirteen, won his first battle at fifteen and became head of the clan at eighteen, but his mother Yoshihime never approved of him due to him losing an eye to smallpox as a child. When he came to power, he decided that the Date clan shouldn't be so submissive so he went on the offensive, conquering the neighboring clans. This ended up sorta biting him in the ass as one of the clans, the Hatakeyama, decided to take his father Terumune hostage to force him to pull his forces back. When Masamune confronted the Hatakeyama, Terumune instead told Masamune to just open fire and kill him along with the Hatakeyama. Masamune did so... and then hunted down the rest of those Hatakeyama supporters and tortured them as retribution.

With his father gone, Masamune continued his conquest, hindered only by a power struggle within his family: his mother who favored his younger brother Masamichi allegedly went as far as poisoning him. In the end Masamune killed his younger brother and disinherited him for seven generations(!) so no descendant of his would be a threat to Masamune's and his heirs' position. Yoshihime fled back to her home clan and her son never saw her again (which apparently did wonders to their relationship, as they seem to have reconciled with time).

Masamune succeeded in unifying the north, but by the time he did so, Toyotomi Hideyoshi had already conquered most of Japan, so Masamune had no choice but to become Hideyoshi's vassal (though he was rather unenthusiastic about it). Under Hideyoshi's banner he took part in in eliminating the Hojo at Odawara and was rewarded, moving to Sendai (in the region that was recently ravaged by the earthquake/tsunami) as his home base and turning it from a small rural community into a prosperous city. He eventually allied with Tokugawa Ieyasu against an aging Hideyoshi, and aided him in seizing the power after Hideyoshi's death. It took a long time for him to give up his dream of ruling Japan, but after he realized he had no chance, he was a loyal and steadfast ally of Ieyasu. Although considered a Wild Card and not really trusted by anyone, Masamune did prove his loyalty and even recited a poem when Ieyasu was on his deathbed.

He kept several Christians under his protection (his favourite daughter may or may not have converted) and encouraged travel to foreign countries, even sending a letter to the Pope trying to establish ties for a potential alliance. He was also greatly interested in cookery and art, and possessed a rather flamboyant taste, leading folk etymology to assign the kanji of his surname to the word "date" (approx. "flashy").

His lasting success, along with his unique personality, mean Masamune has remained as one of the more popular historical figures in modern Japan.

Of course, not to be confused with that sword from the Final Fantasy series. That's actually named after a different Masamune

Tropes associated with the historical Masamune:

Bling of War: His pimped out army became a public spectacle as they traveled from the north to Kyoto to prepare for the invasion of Korea.
Bunny-Ears Lawyer: Despite his quirky nature he was an effective leader and administrator.
Cain and Abel: Ended up killing and disinheriting (in that order) his own younger brother to put an end to a power struggle within the family.
Can't Hold His Liquor/The Alcoholic: He was rather infamous for his low alcohol tolerance, but this didn't stop him from drinking, nor from messing up by deciding on matters while under the influence.
- Once he faked illness to avoid meeting the shogun, Tokugawa Hidetada - in reality he was nursing a hangover from hell.
Eyepatch of Power: Subverted - he always wears an eyepatch in fictional depictions, but there's no record of him wearing one in real life. In fact, he specifically requested that his portraits be painted with him having both eyes. The reason for the latter is a bit of Values Dissonance in the West—in East Asia the body is considered a gift from the parents, and alienating any part of it, even due to reasons beyond one's control, is considered disrespectful.
Headbutting Heroes: He was famously on bad terms with Naoe Kanetsugu.
Not-So-Fleeting Passionate Hobbies: A man of many interests, he had a great many hobbies, some of which were customary for men of his status, such as poetry and appreciation of art; and some were way out there for the age, such as cooking and taiko drumming.
Cool Helmet: His iconic crescent moon helmet.
Real Men Cook: One of his many hobbies was cooking, and he liked to entertain his guests by serving them food he made himself.
Sadistic Choice: When forced to shoot through his father to kill the kidnappers.
Subversion of Self-Made Orphan

Works that references Masamune:

NHK drama film Dokuganryuu Masamune, starring a young Ken Watanabe. Obviously about Masamune.
Samurai Warriors included him in the first game as a Bratty Half-Pint who'd interrupt nearly every battle and beat the crap out of both sides. He's Dual Wielding two bokkens in that game. From the second game onwards, he got a Plot-Relevant Age-Up and now wields a saber in one hand, and two pistols on his side. His focus changed about finding the right time to fulfill his ambition of taking over the land for himself and then make it prosper.
- However, in Warriors Orochi, he's back, and now constantly being a henchman of Orochi.
Sengoku Basara's version has attained infamy, wherein Masamune is an Engrish-speaking Badass who can wield six swords at once and is a brazen cool dude who is the main protagonist and rival of Sanada Yukimura. And his horse is a motorbike.
Masamune: Are you ready guys?! Put ya guns on!
Samurai Deeper Kyo included Masamune who uses the nickname 'Bontenmaru' (historically his childhood name), and while a Boisterous Bruiser and ambitious, he remained as an ally of Kyo. Also he's voiced by Norio Wakamoto.
In Sengoku Rance of the Rance Series, Masamune is a literal one-eyed demon that looks kinda like a Peeping Eye.
While not appearing directly, in Persona 3, the main character's historical teacher Mr. Ono is an obvious Date Masamune fanboy. He teaches his class wearing Masamune's helmet (and probably eyepatch too) and thinks history lesson isn't cool until you get into the part about Masamune.
Sengoku Otome give us a gender flipped version of Masamune. Who's actually Hideyoshi's teacher in the present day, Date-sensei its later revealed that Date Masamune is her ancestor and that she and Hideyoshi end up in a world where Date Masamune did not exist for some unknown reason.
Date Seiji/Sage Date of Yoriden Samurai Troopers (Ronin Warriors) is explained to be a descendent of Date Masamune in supplementary materials.
Total War: Shogun 2 has him as a special general for the Date Clan.
Gate 7, CLAMP newest manga, give us a grade student as the newest version of Date Masamune. And not just that; he's a yandere crazy jealous guy towards the main character, Hana, is a bratty half pint towards anybody else and tried to kill the poor naive newcomer twice. But still, when he's in the battlefield, he's a hell of a good swordfighter.
Masamune Date is mentioned in Yaiba: apparently he was the owner of the "Victory" Orb, required to discover the location of the legendary Orb of Ryuujin. He hid the orb in a small temple.... along with other 99999 fake ones.
A Gender Flipped version appears in Oda Nobuna No Yabou. She's a Large Ham Bratty Half-Pint here too.
He's sort of the main character in Tono To Issho, where he's an idiotic Man Child with an eyepatch fetish.
Another Gender Fliped version appears in Sengoku Collection, where she got involved with the Yakuza and is currently on the run from police.
Masamune appears as one of the major warlords of Pokemon Conquest. He controls the nation of Avia and has a Braviary as his perfect link.