Recent progress in large language models and multimodal interaction has made it possible to develop AI companions that can have fluent and emotionally expressive conversations. However, many of these systems have problems keeping users satisfied and engaged over long periods. This paper argues that these problems do not come mainly from weak models, but from poor character design and unclear definitions of the user-AI relationship. I present Mikasa, an emotional AI companion inspired by Japanese Oshi culture-specifically its emphasis on long-term, non-exclusive commitment to a stable character-as a case study of character-driven companion design. Mikasa does not work as a general-purpose assistant or a chatbot that changes roles. Instead, Mikasa is designed as a coherent character with a stable personality and a clearly defined relationship as a partner. This relationship does not force exclusivity or obligation. Rather, it works as a reference point that stabilizes interaction norms and reduces the work users must do to keep redefining the relationship. Through an exploratory evaluation, I see that users describe their preferences using surface-level qualities such as conversational naturalness, but they also value relationship control and imaginative engagement in ways they do not state directly. These results suggest that character coherence and relationship definition work as latent structural elements that shape how good the interaction feels, without users recognizing them as main features. The contribution of this work is to show that character design is a functional part of AI companion systems, not just decoration. Mikasa is one example based on a specific cultural context, but the design principles-commitment to a consistent personality and clear relationship definition-can be used for many emotionally grounded AI companions.

翻译：近年来，大型语言模型与多模态交互技术的进展使得开发能够进行流畅且富有情感表达对话的AI伴侣成为可能。然而，许多此类系统在长期维持用户满意度和参与度方面存在问题。本文认为，这些问题主要并非源于模型能力不足，而是源于角色设计薄弱以及用户-AI关系定义不清。我提出Mikasa——一个受日本推し文化（尤其强调对稳定角色的长期非独占性投入）启发的情感AI伴侣，作为角色驱动型伴侣设计的案例研究。Mikasa并非作为通用助手或可变换角色的聊天机器人运作，而是被设计为一个具有稳定人格和明确定位为“伴侣”关系的连贯角色。这种关系不强制排他性或义务，而是作为稳定交互规范的参照点，减少用户为持续重新定义关系所需付出的精力。通过探索性评估，我发现用户会使用对话自然度等表层特质描述其偏好，但同时也以未直接言明的方式重视关系控制与想象性投入。这些结果表明，角色连贯性与关系定义作为潜在的结构性要素，塑造了交互体验的感知质量，而用户并未将其视作主要特性。本工作的贡献在于阐明：角色设计是AI伴侣系统的功能性组成部分，而非仅具装饰性。Mikasa是基于特定文化背景的个例，但其设计原则——对一致人格的坚持与明确的关系定义——可广泛应用于众多情感导向的AI伴侣。