>>532041059
Koyanskaya is partially formed from the vengeful spirits of the animals killed by the Tunguska event. As a result she quite likes animals and animal-related servants. She's also just particularly fond of cute and innocent types.

Beni-Enma is both cute and innocent, as well as an animal (a sparrow specifically) so she uniquely hits a double whammy for Koyanskaya.

This is your reminder that Koyan Dark gives a 30% ATK and DEF up for Animal Characteristics Servants. Though it seems her definition is somewhat loose, as it includes servants wearing bunny suits like Rulertoria and Bunyan.