I am a Piano by Asumiko Nakamura - oneshot
It's a journey of a piano represented by a girl. It's just 8 pages and yet it draws lengthy reviews from readers. Although I've read it like under 5 or 10 minutes it took me more than a few days to decide if anthropomorphic characters count for reverse harem. Just read it and be captivated as well. Thanks to Tartza for the tip!