343 Guilty Spark is the moniter of installation 04. He appears to be your ally in Halo 1 but tries to kill you at the end of the Halo 1. He survives the explosion of 04 and comes back for Halo 2. A good guy now. On Halo 3 he again returns but kills Seargent Major Johnson in the end, but Master Chief destroys him. The sentinels are another part of the forerunners. They help protect the moniter and all that happens on on Halo. They also prove a effective weapon against the flood.
It was brough to my attention that I forgot to give credit for the sentinel that was not my design. So thanks a bunch Ryan Jackson. I apologize for any inconvieniences this might have caused.