Obesity is a major clinical problem for individuals and health care systems worldwide, alarmingly fuelled by body mass excess in the juvenile age. In spite of its multi-factorial origin, unhealthy lifestyles relative to alimentary behaviours and physical activity habits play a major causative role. Thus, an important preventive action of this condition can be conducted by fostering motivation of young people towards healthy lifestyles through engagement and inclusion. ICT technologies offer a powerful tool to address effectively this serious medical and societal issue by the development of persuasive strategies based on an accurate modelling of individual's characteristics. PEGASO is a technological multidisciplinary project aimed at promoting healthy lifestyles among teenagers, through assistive technology enhancing motivation to healthy lifestyles, empowered by a virtual individual model (VIM) for user characterisation. The VIM intended for the PEGASO project, including functional, physical and psychosocial aspects profiling young individuals' health status and behaviours relevant in alimentary and physical activity domain, will enable the development of an individualised assistive technology expected to leverage motivation to healthy lifestyles through implicit and explicit interaction.