Physicists engage with the public to varying degrees at different stages of their careers. However, their public engagement covers many activities, events, and audiences, making their motivations and professional development needs not well understood. As part of ongoing efforts to build and support a community in the informal physics space, we conducted interviews with physicists with a range of different experiences in public engagement. We use personas methodology and self-determination theory to articulate their public engagement motivation, challenges, and needs. We present our set of three personas: the physicist who engages in informal physics for self-reflection, the physicist who wants to spark interest and understanding in physics, and the physicist who wants to provide diverse role models to younger students and inspire them to pursue a science, technology, engineering, and mathematics career. Needs covered a range of resources including science communication training, community building among informal physics practitioners, and mechanisms to recognize, elevate, and value informal physics. By bringing user-centered design methodology to a new topical area of physics education research, we expand our understanding of motivations and needs of practitioners in physics public engagement. Therefore, departments, organizations, and institutions could draw upon the personas developed to consider the ways to better support physicists in their respective environments.