This is a bit of a strange ask, but I'm still confused over Usagi's last name. Some tell me that is Miyamoto his last name and that Usagi is his first name... but then others tell me that Usagi is the family name 😂
Gelu. You have no idea how long I've been waiting for this ask. I have been preparing for this very moment. I'm ready to answer this and I already know I'm going to fail miserably at explaining. Here goes.
Usagi Yojimbo. In the comics, "Miyamoto" is the surname / family name, we know this because his father is Magistrate Miyamoto. This makes our titular character Usagi, his first name.
Usagi Chronicles. In this animated series, we know that Yuichi is descended from Miyamoto. However, his name isn't "Miyamoto Yuichi", Yuichi is in fact his surname / family name. That means they both share the same first name, Usagi.
In Japanese fashion, the family name comes BEFORE the first / given name. Let's take George Washington for example. To do so properly, it would be "Washington George". So if we see "Miyamoto Usagi" and "Yuichi Usagi", that means their last names are in front of their first names. If you watch any Japanese shows or movies, they introduce themselves with their last name first and given name after, and people will call them by their last name. In TMNT 2003, he specifically introduces himself (and we will ignore the poor pronunciation of Japanese words including his own name) as "Usagi Miyamoto". This is so that the Western audience knows Usagi is his first name, Miyamoto last. (Also told the audience his last name was Miya-ma-to but uh. We're ignoring it, right?) This is not the proper way for him to introduce himself if he were being strictly Japanese… we say it's fine because it was for an English audience at the time, but this is the only time (that I can recall) his name being introduced in this way.
Typically in Japanese fashion, when you first meet someone or are on acquainted terms, you would only refer to them by their last name and add "-san" to the end, until you become closer and can use a different honorific or use their given name. However, all of the Usagi series have been adapted for western consumption, so they use their first names in all of this familiar media. The western version of "formal" or "polite" in a Japanese setting is taking the first name and adding "-san" to the end instead. So in 2003, Usagi calls Leo "Leonardo-san", and Leo calls him "Usagi-san" back. In 2012, the same thing. When you meet someone who is older than you in the west, you would usually say "Mr./Mrs./Mx. Lastname". That is the English equivalent of what "-san" is, loosely. I'm not sure why western media wants to flip names and add -san to it anyway, since we do use last names sometimes? Mr. Obama… Miyamoto-san. We can also chalk this up to relationships, as Leo and Usagi have been close in every iteration, perhaps they just went to the "first-name-basis" immediately. (copium)
I don't know if you've noticed, but there are a lot of Usagis! Even just 2 is too many, and I have 4! (Well, one was married in, but still.) In my case, I wanted to be able to have multiple samurai rabbits out in the world of Rise, but I couldn't have them all named "Usagi" unless that was all their last name-- which meant they all also had to be directly or indirectly related to each other. Which posed other problems, like… what about Magistrate Miyamoto?? Well.. he's still related, just differently. I know some important plot points get washed out when you move characters and their specific roles around, but that's exactly what ROTTMNT does, and excels in. So we do some problem solving, we write every idea down, and then we iron out the edges.
I am not an expert in the naming culture of Japan, though I can understand why people might be confused or spread misinformation as though they are correct. It takes just a little bit of reading and context to understand, which I hope I've been able to provide. TBH I still get confused, but that's because I am actively doing it differently by going against what it normally is.
TL;DR - Usagi is the first name in all published media, with Miyamoto and Yuichi both being family names. The proper way to introduce oneself in Japanese is [Last Name] [Given Name], which Miyamoto Usagi and Yuichi Usagi both follow this rule (and are both Japanese). In my fanon, I have too many Usagis in one time period, so Usagi is the family name and everyone can still keep their original name. Hope this helps!