User blog comment:Omega natsu2/Episode 182: The Burning Earth, Review./@comment-24748439-20140517125916/@comment-4829046-20140517181251

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Onii-chan is not younger brother. That's ototo and in Japanese honorifics, the younger is not addressed the same way as the older. There is no addition to the name when referring to a younger person. You just use it instead of their name, not in conjunction unlike with elder.