Interventions can foster personal growth. However, our understanding of the specific mechanisms for change and the types of interventions driving this growth process remains limited. In this study, we focused on emotion regulation ability as a potential mechanism. We examined the effects of an affirmation coaching intervention on changes in emotion regulation ability, an important facet of personality. In this coaching intervention, participants created a personal mantra/goal derived from a selected image and positive associations linked to this image (motto goals). This is considered to enhance emotion regulation abilities by internalizing self-stabilizing value. We assigned sixty-six participants to either this affirmation coaching intervention or one of two control coaching interventions: specific-goal versus indulgence coaching. Before and after each intervention, participants completed questionnaires. Only the affirmation coaching intervention significantly increased in adaptive aspects of personality. Notably, the affirmation coaching intervention increased emotion regulation ability, and this effect persisted even when controlling for extraversion and neuroticism. Furthermore, exploratory analysis showed that extraversion increased following the affirmation coaching, while neuroticism remained unchanged. Our results suggest that emotion regulation ability might be the key factor in personality growth. It could be more malleable and/or respond more strongly to short-term coaching, compared to neuroticism. Thus, the malleability of personality traits may not be an all-or-nothing phenomenon; rather, it could depend on the facet of emotion regulation ability. We discuss potential mechanisms of personality growth, distinguishing between emotion regulation and emotion sensitivity.