Kono Oneesan wa Fiction desu!?

Late one night when Jun was out on his bike, he came across a drunk, half-naked girl sprawled on the side of the road. She came round just long enough to be uncomfortably affectionate. Jun took her to his home so she could sleep it off. It turned out that she is Kawase Narumi, his author mother's new assistant. Unfortunately, she has no idea what she's doing in her new job, so Jun's going to have to coach her. He's also going to have to deal with her habit of getting drunk, mostly naked, and cuddly. Jun's life will never be the same!

(Source: MangaHelpers)