From Destinypedia, the Destiny wiki
Obsidian is a Ghost on Mars. He ran into Cassiopeia as she took scans of the surrounding area and was mistakened for Obverse, another Ghost. He caught her up on recent happenings in the Last City, including the knowledge that there were more Guardians than ever. After both encountered a third unidentified Ghost Obsidian wished Cassiopeia well and moved on.
List of appearances