As he campaigned to be South Korea's president, Moon Jae-in was often portrayed in Japan as a radical leftist, perhaps even propped up by North Korea (though evidence never went further than paranoia-inflected hearsay). It was assumed that the 2015 "comfort women" agreement would surely be scrapped and his overtures to North Korea would empower the North, if not signify outright capitulation.
But far from being a waffling dilettante or leftist firebrand, he's shown himself to be a canny leader who's focused on the long game. Notably, his initial statements upon taking office regarding the Terminal High Altitude Area Defense (THAAD) system and the comfort women agreement were measured and rarely went further than promising to review the prior commitments made by his predecessor, Park Geun-hye. Given the scale and depth of Park's corruption scandal, possibly touching every policy initiative of her administration, it's fair to undertake a thorough review of its major decisions, if only to legitimize future initiatives.
For Japan, Moon's success in office will largely depend on how he manages historical issues almost as much as his management of the North Korean threat. That's not to say that history issues are irrelevant to South Koreans — a poll conducted one year after the 2015 agreement showed that 59 percent of South Koreans thought the agreement should be scrapped, with only 25.5 percent in favor of keeping it — but Japan's response to the placement of a comfort woman statue outside of its consulate in Busan showed how much weight the Japanese government places on history issues in its bilateral relations with South Korea.