Previous studies have examined gender differences in learning through digital games (Hayes & King, 2009; Joiner et al, 2011; Hartmann & Klimmt, 2016; Lawrence et al, 2018). Thus, specific preferences of girls should be considered when developing a serious game addressing a female target group. Especially, avatars and non-player characters present key features for game experience and learning objectives of a serious game (e.g., Lee & Hoadley, 2007). Based on a project in which a serious game for girls was developed, we will analyse the research question, which design and style elements of female avatars determine preferences especially for girls, and what kind of non-player characters (NPC) do girls prefer as feedback providers. Since the game was developed to be used at schools, we, additionally, analysed if girls’ preferences on avatar and NPC design are acceptable for boys as well. We conducted two studies with girls and boys age 12 to 16 (N=116). In the first study, participants could decide on different female avatars (face and body) and style elements (haircut, glasses, makeup, skin colour, earrings, head shape, body weight, body height). In the second study, they could name preferences for non-player characters as feedback providers for a technical problem. Results showed differences for avatar choices and style elements: while girls preferred glasses, a little bit of makeup, and a more normal-weight-looking body, boys preferred thinner bodies and an avatar with even less makeup. Preferences of non-player characters also varied between boys and girls. While girls preferred male and female characters as feedback providers, boys mainly chose male feedback providers. The results are discussed on the background of game design. Furthermore, we will present how the results informed the design of the serious game Serena Supergreen and The Broken Blade.