More Japanese language headcanons for the TMNT boys that nobody asked for!
These are my head canons of how the TMNT boys address each other when speaking Japanese.
I see much of the fandom, particularly fans of the 2012 series, have the younger turtle brothers refer to Leo directly as お兄さん (onii-san) and often have the older brothers refer to the younger ones as 弟 (otouto) in fan fics and it's had me thinking more about how I think the turtles would interact with each other in Japanese. お兄さん (onii-san) is very formal and actually creates distance between siblings, implying they do not have a close relationship when used. It’s usually used to refer to another person’s older brother, or sometimes to talk about your own brother to a third party. 弟 (otouto) is never used to directly address one’s younger brother. So here are my own personal head canons of how I think the turtles would actually address each other in Japanese.
I 100% believe that Raph and Donnie would call Leo 兄貴 (aniki) when speaking directly to him. 兄貴 (aniki) is interesting, because those somewhat familiar with Japanese media might recognize this word as one used by Yakuza, the Japanese mafia, to refer to those in a higher position than them. However, it is also used by close siblings either seriously or as a kind of joke. I think Raph and Donnie would use it for Leo both ways depending on the situation. And I think Donnie would use it for Raph, too. As for Mikey, I think it depends on the version. Most Mikeys would probably use 兄貴 (aniki) too, except for 2012 Mikey. He seems like he would be more likely to use 兄ちゃん (nii-chan) to refer directly to each of his older brothers. Dropping the honorific お (O) at the beginning and adding -chan instead of -san is a much more natural way for a younger brother to address an older brother he has a close relationship with. It also sounds more innocent and child-like coming from the speaker which is why I think 2012 Mikey is most likely to use it, but I think 2003 Mikey would use it when trying to suck up to his older brothers to get what he wants or get away with something. I think 2003 Donnie might occasionally use 兄ちゃん (nii-chan) for Raph because they seem to have that kind of relationship with Donnie calling Raph, “Raphie”, in the show.
How older brothers refer to younger brothers is more simple. They would likely use the brother's name or nickname followed by the suffix ～ちゃん (-chan), or simply use their name without a suffix like we do in English. I think Leo is most likely to use ～ちゃん (-chan) when speaking to Mikey. Sometimes he'd use it with Donnie, but never Raph. Raph is always just Raph.
Raph always adds ～ちゃん (-chan) to Donnie and Mikey's names when speaking to them in Japanese, and Donnie always uses it to refer to Mikey.
They all address April as 姉ちゃん (nee-chan meaning older sister) when speaking to each other about her. She doesn't speak Japanese, but she asks Splinter one day why they sometimes call her Nee-chan, and she is delighted to learn they think of her that way.
Anyway, I'm by no means saying don't use Onii-san and Otouto in your fics or anything. I just wanted to provide some more natural alternatives for anyone out there interested in sprinkling a little authentic Japanese into their fics to keep bilingual readers invested in the story. Do with this info what you will.
