In terms of size and mass, Hiisi is somewhat comparable to Saturn, in that it is rather smaller than Jupiter and much less massive. This means that, like Saturn, the planet has a comparatively low density. Unlike Saturn, Hiisi's upper atmosphere appears to be relatively clear of dense cloud cover, and water vapour has been detected within that atmosphere.
Hiisi's name was assigned as part of the international NameExoWorlds project. Names for the starHAT-P-38 and its planet were chosen by the people of Finland, who gave the name Horna ('hell') to the star, and Hiisi to its planet. Hiisi is a name with a complex etymology, originally describing nature spirits and sacred places, though in combination with Horna it more specifically suggests evil spirits or devils.