Day Walker is a Manga/Manhwa/Manhua in english language, Adult series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.cc fastest updating comic site. The Summary is

Day Walker Summary: National Intelligence Service black ops agent ‘Suhyeok’ is tasked with assassinating ‘Nanayama,’ the Yakuza leader heading a drug trafficking organization in Korea. To his surprise, the target is a captivating woman named ‘Anna,’ who appears disconnected from any violence. Sensing something amiss with his mission, Suhyeok embarks on a path of vengeance against both the National Intelligence Service, which has betrayed him, and ‘Nanayama,’ who massacred his comrades.