User blog comment:B214/High School DxD EX/@comment-104.178.60.59-20160120185935/@comment-27232228-20160121163904

The three strongest siblings each have their own type of leadership in the team. I think it is better to explain it like this:

Ex is the main hero and the face leader since all the siblings acknowledge his power (he is the eponymous character of this story, after all), but he is reckless and sometimes cannot make a good decision (the biggest offense is when he activates his Counter-Balance for fighting a lackey instead of saving it for fighting Loki).

Airi is number two (second-in-command, second oldest, second strongest), AKA The Lancer, and she is the figure who can calm Ex down and IMO have the best coordination with all the siblings.

Kurenai is the acting leader and the most intelligent, with a quick wit and excellent improvisation. He is the eldest and the wisest, so all the siblings refer to his decision.