AT21

"look at theese people, mostly just Animals if I do say so Myself, These guys are called AT21, a group of bastards who are the rivals of BT21, nice view".

--Su Ji-Hoon, War and Peace at the Base Camp

AT21 are the Rivals of BT21.

in When the Cold Breeze Blows Away, they are members of the Coalition of the Red Star.

TUTU, MOYA, CHIEF, GWANG and SPOOKY are the only members of AT21 that appear in LINE Hello BT21.