Undercover is a Manga/Manhwa/Manhua in english language, Manhwa 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

The National Intelligence Service field agent, Kim Sijun, disguises himself as a priest to infiltrate a Russian church. There, he meets Anna, a Korean orphan, and their connection blossoms into first love. However, every time he looks into Anna's eyes, a strange sense of déjà vu starts to awaken forgotten memories from his past. Alongside this innocent first love, the target of the current operation gradually begins to reveal itself beneath the surface. To stop the illegal arms deals of the Russian mafia and protect Anna, Sijun must confront his past.