Rhianna transformations seem to cost 2 actions. Tested on a new game with each of the four available, causing 5 to go to 3 if you ignore the Akarion and Rhianna scene, and 4 to 2 if you interject. Doing a second transformation still has the same effect, and this double reduction still applies even on other rounds (tested by not talking to Rhianna or even going to areas she's at until round 2. By the way, you can wait to do the Love at First Sight thing by doing exactly that, if anyone else wanted to know that.)