Stable personality is a key requirement for personal AI assistants, yet how reasoning models acquire such traits remains underexplored. Existing work largely treats personality as role-play and inherits the standard reasoning-model recipe: SFT warm-up followed by RLVR. We ask whether this recipe is appropriate for natural personality acquisition, where a trait should shape both final answers and intermediate reasoning, persist under prompt variation, and preserve general capability. Holding Stage 2 fixed as rubric-based GRPO, we vary Stage 1 on Qwen3-8B across four Big Five trait directions and compare standard SFT with MCQ-GRPO, a simple on-policy warm-up over the model’s trait-relevant choices and reasoning traces. We evaluate each endpoint using an LLM-judged Rubric for stylistic expression and TRAIT for introspective disposition. We find that SFT-then-GRPO achieves high stylistic scores but is weaker on the introspective probe. In contrast, MCQ-GRPO-then-GRPO yields stronger introspective acquisition at a smaller mathematical-reasoning cost. Weight- and activation-space analyses further support this finding, suggesting that personality acquisition is not only a question of how much trait is displayed, but where it is carried.