Downsized from a four-piece, Fifty Fifty's last remaining member Keena celebrated the group's first anniversary with an apology.

Keena penned a hand-written letter to fans Saturday, a year on from the group's debut.

"I'm cautious as to how to start this since I'm addressing my fans, Hunnies, for the first time in a while, but I'm plucking up my courage to speak on my honest thoughts," Keena began her letter.

"First of all, I'm sorry to Hunnies who have gone through a hard time due to my indiscreet actions and I am thankful to them for enduring through this indefinite period for me," she wrote.

Keena, along with her three bandmates of Fifty Fifty, had begun a legal battle against the group's agency, Attrakt, in June, requesting an injunction to terminate their exclusive contracts. The Seoul court rejected their request in August and the four members appealed. Attrakt in October terminated the contracts with Saena, Sio and Aran, but not with Keena, who withdrew her appeal at the last moment.

"I've learnt, experienced and matured a lot in the past year. It was my memories with Hunnies that have pulled me up every moment," she continued in the letter, adding, "I promise to make you guys happy again as Keena of Fifty Fifty with good performances and music."

Debuting under Attrakt, Fifty Fifty gained enormous popularity with its mega-hit single, "Cupid." With the song, the quartet became the fastest K-pop group to make it onto Billboard's Hot 100 song chart, and "Cupid" went on to become the longest-charting song on the US singles chart and the British Official Single Chart.

Keena is set to attend the Billboard Music Awards ceremony at the MGM Grand Garden Arena in Las Vegas on Sunday.

Fifty Fifty has been nominated for the top duo/group and top global K-pop song award categories of the annual music event.