弟

1. Origins, structure, and basic meaning

• Structure: 弟 (おとうと) is a kinship noun for a male sibling younger than the speaker or other reference person.
• Basic usage: 弟と一緒に歩く marks the younger brother as a companion, while 私の弟は料理が好きだ identifies a particular sibling through possession and makes him the topic.

2. Common pitfalls and similar expressions

• Reference point for the age relation
  • The “younger” relation in 弟 is relative, not an absolute age category. 私の弟 states the reference person as 私, while the first example leaves that reference to context.