Yang Hong Seok (양홍석) is the lead vocalist of Pentagon under ANDMARQ. Hongseok has lived in San Diego, Singapore, and China but has since then stayed in Korea to pursue his dream of becoming an idol. Hongseok was first known for appearing in YG Entertainment's survival show, MIX&MATCH.
In 2014, Hongseok joined YG Entertainment and participated in the reality survival program Mix & Match as a member of TEAM B. He was eliminated in final episode alongside Jung Jin-hyeong, failing to debut as a member of iKON. In March 2015, it was revealed that he left YG Entertainment.
In late 2016, he participated in the Mnet reality survival show Pentagon Maker, ultimately making it to the final lineup of Pentagon.
Hongseok debuted as a member of PENTAGON on October 10, 2016 with the mini album "PENTAGON".
In November 2017, Hongseok made his acting debut in the movie "The Love That's Left".
On November 6, 2023 Cube announced that Hongseok has decided not to renew his contract with them. He is still part of Pentagon but no longer with Cube entertainment.