The Klingon security chief on the Enterprise-D is Worf. He is a Starfleet officer who serves under Captain Jean-Luc Picard and becomes the ship's formal chief of security during the early seasons of The Next Generation.
Worf initially joins the Enterprise-D as a relief flight-control and tactical officer in the series premiere, "Encounter at Farpoint." After the death of chief of security Tasha Yar, he becomes acting chief of security and is later formally appointed to the position. His Klingon heritage, strict sense of honor, and experience as a warrior strongly influence how he approaches Starfleet duty.
The alternatives are notable Klingon characters but do not fit the question. Gowron becomes chancellor of the Klingon Empire, Martok is a major commander and later chancellor, and K'Ehleyr is a diplomat with a personal connection to Worf. Worf later transfers to Deep Space Nine, where his story expands into Klingon politics, family, honor, and his role in the Dominion War.