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，而是源于角色设计不佳以及用户与AI关系定义不清。我提出了Mikasa，一个受日本推し文化——特别是其对长期、非排他性地投入于一个稳定角色的强调——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。相反，Mikasa被设计为一个具有稳定人格和明确界定为“伴侣”关系的连贯角色。这种关系不强制排他性或义务，而是作为一个参照点，用以稳定互动规范，并减少用户为不断重新定义关系所需付出的努力。通过一项探索性评估，我发现用户会使用对话自然度等表层品质来描述其偏好，但他们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，塑造了互动体验的感知质量，而用户并未将其识别为主要特性。本工作的贡献在于表明，角色设计是AI伴侣系统的一个功能性组成部分，而不仅仅是装饰。Mikasa是基于特定文化背景的一个实例，但其设计原则——对一致人格的坚持与清晰的关系定义——可适用于许多基于情感基础的AI伴侣。