2012: My First June, 1&1 and South Korean debut

Juniel released her second Japanese single, "Sakura: Todokanu Omoi" (さくら～とどかぬ想い～), on 15 February 2012.[19] The song was used as an ending theme for various television shows, such as Music B.B., Music Focus and King Kong no Arukoto Naikoto. She appeared on MBC's documentary show K-Pop Star Captivates the World on 22 April 2012,[20] where her hype track, "Fool" (feat. Jung Yong-hwa (CN Blue)), was released.[21]

Her debut Korean single, "Illa Illa", was released on 7 June 2012.[22] She released her first EP My First June and held her debut showcase on the same day.[23] Her self-composed songs, which were previously released in Japan, were also included on the release.[24] "Illa Illa" was later chosen as the theme song for the character Im Mae-ari, played by Yoon Ji-ni, in the popular Korean drama series A Gentleman's Dignity.[25]Juniel made a cameo appearance on the series as a street performer in Hongdae.[26] In August 2012, she was chosen to endorse Bean Pole's sub-brand Bike Repair Shop alongside the indie band Busker Busker.[27]

Juniel's second EP, 1&1, was released on 20 November 2012.[28] 1&1 peaked at number four on the Gaon weekly album charts, and number twenty-one on the monthly charts, selling a total of 3,097 copies. The music video for its lead single, "Bad Man", was released on the same day,[29] peaking at number four on both the Gaon and Billboard Korea K-Pop Hot 100 charts. Other songs included on the album are Korean remakes of "Boy", "Oh Happy Day", "Cat Day" and a new song co-written and co-composed by Juniel titled, "Happy Ending".[28]