If you’re trying to call them what I guess would be the English equivalent of “little bro,” then 弟(おとうと) “Otōto” is wrong because you only use it for actual younger brothers. Instead, you would use 弟分(おとうとぶん) “Otōtobun.” Small difference, but it matters.
Similarly, if you want to call someone “big bro” as a nickname, you use 兄貴(あにき) “Aniki.”
To my knowledge, the word is only used to refer to one’s blood-related younger brother (or the blood-related younger brother of someone else). It sounds strange to call a friend that.
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If you’re trying to call them what I guess would be the English equivalent of “little bro,” then 弟(おとうと) “Otōto” is wrong because you only use it for actual younger brothers. Instead, you would use 弟分(おとうとぶん) “Otōtobun.” Small difference, but it matters.
Similarly, if you want to call someone “big bro” as a nickname, you use 兄貴(あにき) “Aniki.”
To my knowledge, the word is only used to refer to one’s blood-related younger brother (or the blood-related younger brother of someone else). It sounds strange to call a friend that.