Lee and bandmates L and Sungjong formed a subgroup called Infinite F and were announced during the "One Great Step Returns" encore concert. They performed their song "Heartthrob" from the Hi! School: Love On soundtrack. Their first officially released song, "Crazy", was included on Infinite's second album, Season 2. They made their debut in Japan with their first single album, Koi No Sign, on November 19, 2014, following in Korea with their first single album, Azure, on December 2, 2014.
