From the heat coming from Rex, the captain knew exactly what rank he was.
He's merely a Master Immortal Spirit rank.
Even though she was of the same rank, she couldn't fight the Special Voidal Knight directly—the physical gap between Spirits and Voidal Monsters was that far apart. It would take powerful equipment as well as artifacts to eliminate that gap, both of which the captain didn't possess.
So, seeing Rex going straight at the Special Voidal Knight was simply reckless.
Rex managed to tackle the Special Voidal Knight before the two disappeared from sight.
A lush-green mound was in the way, barring the captain from assessing the situation.
"I need to help him," She leaped down and began sprinting towards where Rex and the Special Voidal Knight were. "He's strong enough to tackle the serpent. He must be gifted in strength and thought that he could even match a Special Voidal Knight. Soon enough, he'll realize that he was wrong, and I have to be there to aid him then."