A bit of historical background for those finding this masterpiece for the first time.
Maeda Toshimasu Keijiro was a real samurai of the late sengoku (warring states) era. He was the son of Takigawa Kazumasu, an illustrious samurai general under the first unifier of Japan, Oda Nobunaga. Kazumasu was related to the Maeda clan through a series of cross-adoptions and one of his cousins, the childless Maeda Toshihisa, adopted his biological son as his heir. Thus Takigawa's son became Maeda Toshimasu sometime before 1560.
At the time of the adoption, Maeda Toshihisa was the eldest son and heir of Maeda Toshimasa, the lord of Arako Castle, who was a vassal of the Oda clan, so Toshihisa's adoption of Toshimasu was seen as very beneficial by the at-the-time lower status Takigawa Kazumasu. However, in June of 1560 it was Toshihisa's fourth brother Maeda Toshiie ("Yari-no-Mataza") who distinguished himself in the Battle of Okehazama against Imagawa Yoshimoto as part of Oda Nobunaga's personal entourage. This was Nobunaga's greatest triumph and the battle that completely changed the Japanese landscape, launching the career of the "Fool of Owari" and turning him overnight into the "Demon King of the Sixth Heaven". In August 1560 Toshimasa died and Toshihisa gained Arako Castle. Maeda Toshimasu was suddenly the heir of an important retainer of an up-and-coming sengoku warlord. This started his career as a kabukimono, a samurai who eschewed the norms of samurai society and lived wildly in the lands, taking what they wanted from the hapless populace. Kabukimono were bandits, but they were popular with the common people because their primary targets were merchants (it's pointless to rob poor peasants). Their status as samurai put them way ahead of the merchant class in the old Japanese caste system (merchants were technically a rung below farmers and artisans) so they could do as they wished.
That in itself wasn't seen as a bad thing, especially in Owari where the Oda held their ancestral lands. Oda Nobunaga himself was a kabukimono, as were many in his entourage, like the future Toyotomi Hideyoshi (the second unifier of Japan) and Maeda Toshiie himself. Yes. Maeda Toshiie! Remember that as you read this manga because here he is depicted as a stingy old man always worried about his reputation and lands. In real life, Toshiie was the rowdiest kabukimono in Nobunaga's entourage. Despite that, he gained a reputation in old age as a wise and kind elder statesman deeply trusted by Toyotomi Hideyoshi - enough for Hideyoshi to make Toshiie the head regent for his son Hideyori.
Back to Toshimasu. While he was always a kabukimono in youth what distinguished him from others like Nobunaga, Hideyoshi, and his uncle Toshiie, is that unlike them he never stopped being a kabukimono. Oda Nobunaga made his turn into proper statesmanship after his marriage to Saito Dozan's daughter No-hime at the age of 15 and basically left the kabukimono life after the death of his father Oda Nobuhide two years later. Maeda Toshimasu was born in 1543. This manga starts in 1581. Dude was 38 years old and still living like he was in his teens. In fact, the historical Maeda Toshimasu kept up his kabukimono lifestyle until his first retirement from samurai life in 1590, at the age of 47. After that he became a writer and artist in Kyoto until entering the service of the Uesugi clan. What likely precipitated this situation was Toshiie's takeover of the Maeda clan succession. Toshiie was the one closer to Nobunaga and the one who distinguished himself in Okehazama and beyond. Nobunaga trusted him far more than Toshihisa, who was a distant presence. So when Toshiie asked Nobunaga to let him replace Toshihisa as head of Maeda clan, it didn't take much convincing at all. This instantly turned Toshimasu from clan heir to basically homeless, and most likely "stunted" his growth from kabukimono to proper samurai. Toshimasu himself wasn't that important or notable in real Japanese history. He fought several major battles under the Maeda clan, broke off relations with them in 1590, then entered the service of Uesugi Kagekatsu under his friend Naoe Kanetsugu. When Sekigahara happened, Toshimasu was off besieging a fort away from the main battle. He joined the retreat of the Uesugi and retired after the Uesugi-Tokugawa settlement, passing away in obscurity in 1612.
So now we have to talk about "Maeda Keiji". The character Maeda Keiji was created in the Edo era based on the historical Maeda Toshiharu. The Edo era was an era of relatively long peace after hundreds of years of nonstop wars, so of course the people romanticized chaotic violence. The kabukimono of old became popular heroes, and here was a kabukimono who stayed true to his "calling" almost his entire life. The writers of the Edo era made many, many stories around the exploits of Toshiharu, prefering his other name Keijiro and shortening it to Keiji. Keiji became incredibly popular in kabukimono fiction, the Japanese equivalent of Chinese wuxia (think Suikoden / Shuihu Chuan / Water Margin). This manga itself can be considered a continuation of that proud tradition. Most of the stories are ahistorical, of course, but they were popular so people kept inventing them. As a result, the Keiji of popular Japanese fiction was what Japanese people knew of him up until the bakumatsu (fall of Tokugawa shogunate). It should be noted that Toshimasu fought on the "wrong" side of Sekigahara, so he was also popular amongst those fed up with Tokugawa rule.
During the early imperial restoration era, kabukimono fiction was practically banned. As a result, most Japanese people didn't know much about Keiji until the "sengoku revival" in popular fiction in the 1990s - of which this manga was a major part. This manga in essence revived the popularity of Keiji as a character and made him a feature in a lot of games, anime, and other manga in the 2000s. If you remember facing him as Sengoku Musou's (Samurai Warriors') equivalent of Lu Bu, blame this manga.