Self‐report questionnaires have long been used to assess LLM personality traits, yet they fail to capture behavioral nuances due to biases and meta‐knowledge contamination. This paper proposes a novel multi‐observer framework for personality trait assessments in LLM agents that draws on informant‐report methods in psychology. Instead of relying on self-assessments, we employ multiple observer agents. Each observer is configured with a specific relational context (e.g., family member, friend, or coworker) and engages the subject LLM in dialogue before evaluating its behavior across the Big Five dimensions. We show that these observer‐report ratings align more closely with human judgments than traditional self‐reports and reveal systematic biases in LLM self-assessments. We also found that aggregating responses from 5 to 7 observers reduces systematic biases and achieves optimal reliability. Our results highlight the role of relationship context in perceiving personality and demonstrate that a multi-observer paradigm offers a more reliable, context-sensitive approach to evaluating LLM.