Using the HEXACO Model of Personality, we explored two kinds of ideal partner preferences regarding personality traits, namely, to what extent people prefer partners similar to themselves (similarity preference) and to what extent people prefer partners with more desirable trait levels than themselves (aspirational assortative preference). We conducted five studies (overall N > 900) across four countries (China, Denmark, Germany, and the USA), looking at both HEXACO factors and facets, using both self-report questionnaires and real-life data (personal profiles from a dating website), and comprising both student and more heterogeneous samples. The results provided support for both kinds of ideal partner preferences, with important differences across traits. Specifically, similarity preference was supported by all studies concerning all HEXACO traits, and aspirational assortative preference was supported by all four self-report studies (though not the dating website study) concerning all HEXACO traits except for Openness to Experience. Concerning differences in preferences among the HEXACO traits, similarity preference was particularly pronounced for Honesty–Humility and Openness to Experience, moderate for Agreeableness and Conscientiousness, and less pronounced for Emotionality and Extraversion. Aspirational assortative preference, by contrast, was particularly pronounced for Emotionality, Extraversion, and Agreeableness, moderate for Honesty–Humility, and inconsistent for Conscientiousness.