Friend influence in adolescence is well-documented, but the characteristics that contribute to individual differences in susceptibility to influence are not well understood. The present study tests the novel hypothesis that within a friend dyad, having fewer friends than one’s partner (i.e., relative lack of alternatives) increases susceptibility to influence as it reduces dissimilarity and thereby promotes compatibility. Drawn from diverse California (USA) public middle schools, participants were 678 adolescents (58% girls) in reciprocated friendships that were stable from the fall to the spring of sixth grade (M = 11.53 years old). Longitudinal Actor-Partner Interdependence Models assessed peer influence, operationalized as individual change in the direction of increased friend similarity. Consistent with the hypothesis, partners with fewer friends were influenced by partners with relatively more friends in self-reported social anxiety and somatic complaints, as well as teacher-reported academic engagement and prosocial behavior. Academic engagement was the only domain wherein partners with more friends were also influenced by partners with relatively fewer friends. For those with few friends, conformity (i.e., becoming more similar to a partner) can be an important strategy to promote compatibility for strengthening existing friendships.