On April 15, 2021, an intro video was released on LIGHTSUM's social media announcing their name and logo. Later that day, it was revealed that Cube Entertainment was accelerating preparations to debut the group in the first half of the year.
On the midnight of April 19, 2021, Sangah and Juhyeon were revealed as the group's first and second members, respectively. On April 20, Chowon and Jian were revealed as the third and fourth members, respectively. On April 21, Nayoung and Huiyeon were revealed as the fifth and sixth members, respectively. On April 22, Hina and Yujeong were revealed as the seventh and eighth members, respectively. Finally, on April 23, the group's profile photo was released.
2021: Debut with "Vanilla", fandom name, first comeback
On June 10, LIGHTSUM debuted with their first single, "Vanilla".
On September 17, LIGHTSUM announced their official fandom name to be "SUMIT". The name means "LIGHTSUM and their fans become one through the medium (it) of LIGHTSUM (sum)."
On September 24, a teaser posted via the group's social medias announced they will make their first ever comeback on October 13 with the single album "Light a Wish".