With the rapid advancement of AI, agents are gradually capable of mimicking human behavior and performing complex tasks within digital ecosystems. As the boundary between human and agent behaviors becomes indistinguishable, new challenges arise for governance, trust, and ethical oversight in online environments. This paper introduces the concept of Agent Behavior and systematically compares it with human network behavior, summarizing both into a unified framework called the Network Behavior Lifecycle. Recognizing the limitations of traditional governance approaches, the paper proposes the Agent-for-Agent (A4A) paradigm, which embeds specialized regulatory agents within digital ecosystems to provide adaptive, layered, and resilient oversight. Drawing on institutional governance principles and leveraging advanced AI capabilities, the A4A framework aims to ensure agent behaviors remain aligned with societal values and ethical standards. This work highlights the necessity of robust agent behavior governance to secure the integrity and fairness of future online ecosystems, facilitating the responsible integration of intelligent agents in digital society.