Thesis: A character cannot seem sexy to me if that character is not interested in sex.
Ryuuko, the protagonist of Kill La Kill! is a young lady whose father was brutally slain. She came home one day to find his rapidly cooling body as he bled out. This hardship immediately transformed her from a normal girl into a violent woman who will stop at nothing to get bloody revenge.
I have seen 12 episodes of Kill La Kill! so far. At no point has Ryuuko struck me as sexy, mostly because at no point has Ryuuko showed any desire for sex.