S.O. Silbert is a Detective for the New York City Police Department. He was friends with Detective Sam Burke while he served on the force and was one of the few cops to continue to assist Sam after his dismissal from the force.
Silbert met with Sam and Detective Twitch Williams at dinner burnt down by Jason Wynn where the group bonded. Later during a string of murders of the bums in New York City, S.O. Silbert became one of the main detectives on the case. Silbert allowed Sam and Twitch Williams into the Sarah Frost murder scene where Sam discovered the Serpentine Addendum.
Content taken from Image Comics Spawn Wiki