Han Joon was the only one who was not a five-star. While his strength and expertise warrant him such a ranking, he didn't have the chance to take a tower trial again after he got out of prison.
Fortunately, he was one of Scatach's favored esper. Bearing the goddess's stigma, like Zein, actually protected him more than the other five-star espers.
It wasn't just fortunate for him, however, but also for the dungeon team. Because they had no five-star scout--not even Ron--and they were about to jump into several dungeons blindly.
This was the reason why Han Joon came with the dungeon team; reconnaisance.
The success of a raid was determined highly by the amount of information they could have. For this, most dungeons had preliminary scouting to find out the environment inside the dungeon, as well as the predicted type of beasts. This way, a correct team composition could be sent, and the raid would be more efficient.