Comparable in certain respect to mole-rats or other species that evolved in close complex societies (even by human standards), the Dieikae developed sophisticated Game Theory before written language. Reliant above all else upon cooperation for survival, their prehistoric burrows and warrens scaled up readily into towns, cities, and now a sphere of influence that stretches across a substantial portion of the galaxy.
Individual Dieikae are generally bright and decent, but often come across as slightly abstract or disengaged to members of other races. Anyone who's seen them interact with groups of their own kind--especially in an environment where they're comfortable--however understand that, comparatively, they're downright cold to outsiders.
Community is the lifeblood of Dieikae society, and the common good is its polestar. Though each Dieikae is biologically and intellectually unique, they experience themselves as individuals in ways that members of some insectoid or group-minded species cannot, the average Dieikae has very little in the way of "ego" as humans understand the term. Whatever drive toward individual accomplishment an individual might experience is doubly pressured by cultural and social expectations, which almost invariably subsume the good of the individual beneath the good of society. Pathological selfishness in Dieikae society is a quick trip to loneliness, and while Dieikae don't literally die without companionship, the psychological burden borne by defying both nature and nurture us usually tremendous.
That's not to say that the Dieikae are more interested in getting along than getting things done: despite a hostile environment during their evolution, the historical record suggests that the Dieikae tamed their home world (Dieikos) faster than almost any known species. They are excellent collaborators, and (unlike humans) they experience almost no difficulty negotiating what we might experience as complex collective-action problems. While there may be dissent about the best ways forward, it is rarely if ever fuelled by individual self-interest or moral hazard. When the Dieikae first realized that early industry was imposing ecological burdens on Dieikos, consensus that the problem must be addressed was swift, including the industrialists themselves, and solutions were not far behind. Additionally, war or even individual violence between Dieikae is vanishingly rare, so their intellectual and physical resources were almost never taxed by it before they were exposed to the wider galaxy (and still somewhat rarely since then).
The inner workings of Dieikae society are rarely observed; while other races are permitted upon Dieikos, Dieikae are generally most comfortable having spaces where they can be entirely among their own kind, without any pressure to dial back their private rituals and affections. Because Dieikae in large groups can almost seem to flow into one large organism, aliens generally find they're more comfortable interacting with Dieikae on ships, outposts, and other places where their natural tendencies are somewhat more muted.