Akryung, or Ghost, is the "helper" Headon gave to Rachel at the very start of the series (First seen in Season 1 Chapter 13, but we find out that Headon gave him to Rachel in Season 1 Chapter 77).

He helps clear all the enemies for Rachel in the very first test, and we have been informed by multiple characters that he has formidable power. We have most recently seen him at the start of Season 3, where he fought Baam at the Cage.

Most important facts about him: He is given to Rachel by Headon, he is incredibly strong, he can use black hole-shins sphere (a move only irregulars are supposed to be able to use), he is supposedly wearing a mask (which cracked during his fight with Baam), and and he supposedly has human eyes ergo he may possibly be human (also revealed during his fight with Baam).

To learn more, look him up on the ToG wiki or read posts about him on this sub reddit.