Oryzias latipes (Medaka) is an established vertebrate model for studying developmental genetics, genomics, and evolutionary biology. The physiology, embryology, and genetics of this species have been extensively investigated for centuries. Medaka fish recently attracted attention in the field of social neuroscience. This review introduces recent advances in medaka behavioral studies, focusing on female mating preferences and male mate-guarding behaviors. The medaka female has the ability to discriminate male individuals and prefers to mate with socially familiar males (female mating preference). In triadic relationships (two males and one female), the dominant male remains closer to the female and repels the other male (mate-guarding). Interestingly, mate-guarding blocks female social familiarization of the rival male, which can increase the mating success of the dominant male. Importantly, behavioral analyses using a series of medaka mutants revealed critical roles of neuropeptide neuromodulatory systems in regulating their social behaviors. The extra-hypothalamic gonadotropin-releasing hormone system has a central role in activating female mating preference. The arginine-vasotocin system is required for the emergence of mate-guarding behavior.