Billy Koenig is the (seeming) identical twin brother of deceased S.H.I.E.L.D. agent Eric Koenig, along with living agent Sam Koenig and possibly more, having previously joked that there were thirteen of them. He is also a S.H.I.E.L.D. agent, currently stationed at the Playground, a secret base.
In "Shadows", Trip expresses severe doubts to Skye about him and his constantly growing number of 'brothers', although the rest of team, particularly Coulson, appear to have no difficulties with him. The banter between Billy and Trip continues throughout the series.