Researchers have explored how online communities can be leveraged for peer support, but general disinterest and a lack of engagement have emerged as substantial barriers to their use in practice. To address this gap, we designed Merlynne using the Elaboration Likelihood Model (ELM) and games user research, a serious game that motivates individuals to become peer supporters using the Cognitive Behavioural Therapy (CBT) techniques, through play. We conducted a mixed-methods, exploratory study to evaluate Merlynne’s design and specifically studied the Proteus Effect, hypothesizing that players using a stereotypically helpful avatar would have higher usage rates and a higher change in helping attitudes scores than players using a stereotypically unhelpful avatar. Merlynne had high engagement evidenced by usage rates and meaningful participant responses, and serious game techniques were used as effective cues for motivation. Emerging themes from thematic analysis of semi-structured interviews were supported with usage data and survey responses. We also found that avatar appearance influenced player-avatar connectedness and engagement through the frequency of empathy expressed in solutions. In reflecting on our findings, we discuss design challenges such as Ludonarrative dissonance, designing for emotional fatigue, and players’ overconfidence, and present design considerations such as using avatars to promote empathy for those seeking to motivate participation in mental health support and the use of serious game techniques to encourage participation in health interventions.