Fusian: Multi-LoRA Fusion for Fine-Grained Continuous MBTI Personality Control in Large Language Models

TL;DR

Fusian introduces a two-stage framework that enables fine-grained, continuous control of personality traits in large language models by dynamically fusing multiple LoRA adapters through reinforcement learning.

Contribution

The paper presents Fusian, a novel method combining trajectory collection and RL-based fusion to achieve precise, continuous personality control in LLMs, surpassing existing discrete approaches.

Findings

Fusian achieves high accuracy in personality trait control.

It outperforms baseline methods in aligning with target trait intensities.

Demonstrates effective continuous personality modulation in LLMs.

Abstract

Large Language Models (LLMs) have demonstrated impressive capabilities in simulating diverse human behaviors and personalities. However, existing methods for personality control, which include prompt engineering and standard Supervised Fine-Tuning (SFT), typically treat personality traits as discrete categories (e.g., "Extroverted" vs. "Introverted"), lacking the ability to precisely control the intensity of a trait on a continuous spectrum. In this paper, we introduce Fusian, a novel framework for fine-grained, continuous personality control in LLMs. Fusian operates in two stages: (1) Trajectory Collection, where we capture the dynamic evolution of personality adoption during SFT by saving a sequence of LoRA adapters, effectively mapping the continuous manifold of a trait; and (2) RL-based Dynamic Fusion, where we train a policy network using Reinforcement Learning to dynamically compute mixing weights for these frozen adapters. By sampling from a Dirichlet distribution parameterized by the policy network, Fusian fuses multiple adapters to align the model's output with a specific numerical target intensity. Experiments on the Qwen3-14B model demonstrate that Fusian achieves high precision in personality control, significantly outperforming baseline methods in aligning with user-specified trait intensities.