User blog comment:Emptylord/Custom Champion - Whisper the Mana Wind/@comment-4956864-20120326000510/@comment-3974211-20120326001223

Because she uses mana instead of health. Mana is her primary resource,

I was thinking about adding some percentile costs, maybe to her ultimate and speed buff. But the issue with health costs in the current system is that once you have Hextect Revolver, they serve no point at all. While Whisper does not benefit from spell vamp, she has "mana vamp" built in and so any "health" costs she has could be negated by her passive. Thus, anything short of a percentile cost is pointless.