In my opinion, the primary significance of Kyle's existence is to represent the strength of the white-bearded pirates. Kyle, as the captain of the first team of white-bearded pirates, was able to compete with the three generals of the naval headquarters in the peak war, and even kicked the pheasant away in the peak period, which showed that his strength was good.
Secondly, Kyle exists to fulfill the mission entrusted to him by White Beard. In the battle of the peak, Luffy's superb combat skills shine brilliantly in this war. At this time, White Beard thought Luffy was probably the one who wanted to be the One Piece, so he ordered the crew to defend Luffy to the death. As the most trusted person of White Beard, Kyle is bound to accomplish what White Beard told him, and protecting Luffy has become the mission of White Beard to Kyle.
Although Kyle played relatively few roles in One Piece, he played a great role in the development of the whole plot. As an "immortal bird", there is always a blue flame around you when fighting. It is said that the blue flame is endless and life is endless.