Large language models have achieved strong single-turn language understanding and generation abilities, but long-term agent applications require models to handle dynamic interaction history beyond an isolated inference call. Existing agent memory systems address this need through external storage and retrieval and have achieved promising results. However, many existing frameworks rely on full storage or highly structured memory representations, which can increase cost and noise while disrupting event coherence. Inspired by cognitive views of event organization and adaptive forgetting, we introduce ECO-Mem, an event-centric organization framework for visibility-controlled long-term memory. ECO-Mem segments dialogue into source-linked event episodes and connects them through typed event relations, representing memory as linked events rather than isolated facts. It implements forgetting as non-destructive visibility control instead of destructive deletion. At query time, it traverses the event-relation graph to identify relevant hidden support and reactivates it only when the source evidence confirms the query context. Experiments on LoCoMo and RealTalk validate ECO-Mem’s effectiveness and efficiency for long-term agent memory.