Final answer:
The rhetorical strategy used in the sentence is understatement, a method of presenting something as less significant than it is, often to subtly shift focus or convey a trivializing tone.
Step-by-step explanation:
The rhetorical strategy used in the sentence 'Him? Oh, that's just my brother.' is understatement. This technique involves presenting something as less important, serious, or significant than it actually is to create a particular effect. The speaker is minimizing the presence or importance of their brother, possibly to indicate familiarity, dismissiveness, or to downplay any interest the listener might have. This strategy can convey a casual or trivializing tone, often used when the speaker wants to subtly shift focus away from the subject being mentioned.
In the context of rhetorical analysis, identifying such strategies is important for understanding the speaker's intent and the context's demands. Other common rhetorical strategies include the use of pronouns to create clear and personal writing, employing consistent voice, and maintaining an awareness of the rhetorical situation. Rhetorically effective writing often intentionally meets or challenges conventional expectations.