It's half-size and the right stroke goes down, so it must be a tsu for doubling the sa. The next guess would be a ワ, but I've never seen it half-sized, so I'm sticking with a ツ. But Irassa doesn't make sense either, so either the text continues down her chest, or there's something we're missing here...
Seika said: Irakusa. The source link have the title, which is a story the illustrator wrote (also hosted in Pixiv). :)
Well that explains it. I never would have guessed a ku, though. I wonder what the story's about, but based on the cover, it's likely not going to be a comedy.
