Needless 0 is the Prequal turned Spin Off to Kami Imai's Needless Manga Series. It is composed of only 2 Volumes and 10 Chapters overall. Needless 0 actually predates Needless (Manga) (2004) by 3 years as it was released in 2001, and acted as a teaser of sorts for the 1st volume. The 2nd volume acts as a supplement to the main story.
Spin-Off of Needless. Cain is a descendant of a Needless, little is known of origin but his father was exterminated for being a Needless. Later Cain finds out he is also a Needless and needs to hide this fact or he will be exterminated.
No other works have been connected so far. Originally the Mangaka planned for a 3rd installment to follow the story line of Adam Neuchweinstein and the rest. As of June 2013 the Manga has ended and no further news has surfaced or leaked regarding a Third installment.