Objective <p>Humans form and maintain long-term cooperative relationships, such as friendships, regardless of genetic connections. Evidence indicates that similarities between individuals play a key role in social selection and preferences. This study examined whether friends show similarity in personality traits, moral attitudes, and conformity, and whether subjective similarity correlates with objective similarity based on questionnaire ratings. Japanese female college students (<i>N</i> = 30 pairs) in friendship relationships participated in a face-to-face questionnaire survey as a friend-pair group, and anonymous female participants (<i>N</i> = 30 pairs) completed an online survey as a random-pair group.</p> Results <p>Permutation analyses showed non-significant group differences in similarity in all variables between the friend-pair and random-pair groups (all <i>p</i>s &gt; 0.050). In the friend-pair group, however, associations between subjective and objective similarity were observed: greater perceived similarity in personality was significantly associated with smaller within-dyad differences in Extraversion (<i>ρ</i> = − 0.47, df = 28, <i>p</i> = 0.009) and moral attitudes towards manners (<i>p</i> = − 0.47, df = 28, <i>p</i> = 0.008). These correlational findings, derived from Japanese female college students, suggest a potential association between subjective similarity in personality and moral attitude among friends, while acknowledging possible effects of differing data collection modes.</p>