Kurohime looks like an average black fish, and is female. When she communicates with Masamori, her mouth makes the similar motion of a fish swimming in water. Her fins flap to stay airborne.
It is currently unknown how Masamori gained the ability to summon Kurohime. According to Shimano, Masamori has not completed the Musou training in the family well, which is normally how a Sumimura would gain access to their landlord (as a non-Successor, Masamori was not allowed to attempt the training in the first place). When recalling ability users that may have landlords, Masamori is the first that Yoshimori thinks of, followed by Ito Orihara and Tatsumi Mino (though the latter two's creatures are unconfirmed as actual landlords). 
Power & Abilities
Detection: Kurohime has the ability to detect the location of others (usually Ayakashi, who may or may not be attempting to conceal their presence). To accommodate her, Masamori spreads out his power like a vast ocean, which Kurohime swims through. Presumably the ripples in the 'ocean' bounce off of objects, providing her with the needed information, and Kurohime takes it back to Masamori.
- The word kurohime means Black Princess in Japanese.
- Kurohime is most likely a karasu (Japanese for 'crow' or 'raven'), which is a species of koi fish that is entirely black when viewed from above.