This was largely in part due to Niflheim's actions during their fight against Duke Zareign and the Night Terror Shadow Beasts, as well as their current clash with the shadahis.
Through his quick, well-timed actions and advice, Niflheim saved numerous people from the 9th Division and 5th Unit alike from spiraling into dangerous situations.
The 5th Unit's vanguard, in particular, had formed a great deal of respect towards Niflheim. He was not afraid to place himself in unfavorable positions if it meant easing the burden on his allies. Additionally, whether it was a person from the 9th Division or the 5th Unit, he protected them with the same level of effort and intensity.
Of course, it also helped that Halls and Guan Yu, two trusted members of the 5th Unit, took the initiative to follow Niflheim's orders while Izroth and Lances were preoccupied.