otouto 弟

The word otouto means "younger brother" in Japanese. Sometimes it's translated as "little brother" instead, but it's "younger brother." Even if your younger brother is bigger than you he's still your otouto.

Kanji

The word otouto written with kanji is otouto 弟. It's just a single kanji with a rather long reading.

Originally, the word was oto-hito おとひと, but through u-onbin ウ音便 it became otouto おとうと.^{[弟 - gogen-allguide.com, accessed 2019-11-08.]}

Like most kanji, sometimes it's seen in other words, like kyoudai 兄弟, "brothers," "siblings," or deshi 弟子, "pupil."

Otouto vs. Oniisan

The difference between otouto and oniisan is that otouto always refers to one's younger brother, while oniisan, oniichan, oniisama usually refers to one's "older brother."

So, in a sense, the opposite of oniisan is otouto, age-wise. These words only apply to male siblings, the opposite gender-wise, the words for "older sister" and "younger sister," would be oneesan and imouto.

Note that you can have multiple otouto's, multiple "younger brothers." Because the word otouto doesn't mean "youngest," just "younger."

The "youngest brother" of all your siblings is your mattei 末弟, written with the kanji for "final," matsu 末, probably because he was the last one to be born. You can also simply describe him as ichiban shita no otouto 一番下の弟, literally "the otouto most below (in the age ranking.)"

Otoutosan

You may have already noticed that a lot of family members words in Japanese follow an o__san お〇〇さん pattern, but otouto does not. Why is that? Why is it that oniisan can end in san, chan and sama but otouto can't be otoutosan 弟さん?

This happens because this san stuff are called honorifics, and honorifics are used when you want to make a reverence.

Since your older brother is your elder, and you should treat your elders with respect, the term oniisan gets the reverencing honorifics. But your younger brother is not your elder, he's your junior, so you don't really make a reverence toward him.

One exception is when you're talking about other people's younger brothers. Since it's other people, you might want to speak with respect, so you say otoutosan 弟さん, with a san.

Since you normally don't use honorifics when talking about your own family to other people, otoutosan normally refers to someone else's younger brother, not yours.

Beyond otoutosan, there's also the word goreiteisama ご令弟様, which is an even more polite way to refer to other people's younger brothers, and would be used more in writing.

There's also otoutogimi 弟君, which is old-fashioned, but works just like otoutosan.

Otouto vs. Otōto

The difference between otouto and otōto, with a macron, is merely that it's a different romaji for the same word.

This happens because long vowels, found in syllables like oo おお, ou おう, and oo おー, may be romanized with a macron instead of two vowel letters.