Kira keeps the shard. Her father, Ken Yukimura has the piece altered into a shuriken. Noshiko tells Kira there's a reason Kira kept the obsidian piece. Kira realizes the shuriken represents her first Tail.
In Strange Frequencies, Ken slams and embeds Kira's shuriken into his desk. This action snaps Kira out of a Kitsune homicidal episode.