No. 6 was originally a novel by Atsuko Asano. It was adapted into a manga by Hinoki Kino that ran in Aria magazine from 2011-2013. It was picked up by Kodansha USA in June 2013 and the last volume was published in October 2014.
I really hope No. 6 did well for Kodansha USA. It was a well-drawn title for ARIA. The artist Hinoki Kino now has her own series in ARIA called Setsuna Graffiti. It has two boy protagonists so I wouldn’t be surprised if it was also shounen-ai.(I haven’t read ARIA lately so I wouldn’t know.)
I’m so glad I picked this up. I would have missed a remarkable story otherwise. Now I need to go watch the anime!