Sometimes it really is that simple. Rock Lee, otherwise known as Naruto's resident Krillin, is based on the legendary Bruce Lee. Yeah, the two are each skilled martial artists who practice Jeet Kun Do, and they happen to have the same last name and even kind of the same haircut. But the real cincher here is their birthdays: Both of them were born on November 27. Taken together with everything else, it's tough to call this a coincidence.
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You might not know Hirano Kouta by name, but you might know his manga: Hellsing. His bookish and, let's face it, nerdy-ass face was perfect for High School of the Dead's Hirano Kohta, the series' resident killer otaku. Apparently all you need to steal someone's identity for an anime is change one letter in their name. Coincidentally, I am creating my own anime starring Chris Praut and Barack Obamo.
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A huge part of Attack on Titan's appeal lies in the unsettling nature of the titular monsters. The giants that rampage through populated areas eating villagers somewhat resemble humans, but something about them is a little... off. Their bone structure is exaggerated in weird places, their heads are almost never proportional to their bodies and they have muscular lumps where people usually do not have muscular lumps.

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Historical anime is still anime. While you might see period-appropriate attire and architecture in something like Samurai X, you can still bank on wacky hair and maybe a badass facial scar. Though Rurouni Kenshin isn't based on actual events (what with all the sword magic), the main character is based on an actual samurai. Kawakami Gensai was an assassin who lived in the mid-19th century; when Kenshin creator Nobohiro Watsuki read that Gensai "maintained a duty to his dead comrades," he found inspiration for the aforementioned redhead. Kenshin and Gensai don't have a ton in common other than the fact they're both (former) assassins with a loyal streak. While Samurai X had something of a happy ending, Gensai was executed by his own government after his usefulness had come to an end.

Gensai's legend also inspired a character in another supernatural/sci-fi samurai action anime. Gintama's Kawakami Bansai (was not so subtly) named after Kawakami Gensai. One of the samurai's contemporaries was also honored in Gintama, as the character Okada Nizou was based on the real-life assassin Okada Izo.

Unlike his anime counterpart, Izo was not in fact a blind swordsman who wielded a giant purple sword that turned him into a tentacle monster
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this is a location and not a person. But Saitama's crappy apartment plays a significant role in the series One Punch Man. Especially when the visual comparisons are this strong.
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