We know so far, officially released by disney.
Kion is Simba and Nalas second born cub. While Kiara has the right to rule, Kion is tasked with leading the lion guard. Their duty is to protect and preserve the pridelands. What makes Kions team different is that he is the only lion in the team.
The leader of the group is given the 'roar of the elders' a roar that's much stronger than a lions normal roar. This is speculation but it's probably much stronger for when they get older. After doing the roar, a mark appears on kion that is of unknown origin.
Scar was the leader of his guard. He thought that this power would make him a better king. He orders his guard to kill mufasa, likely to test their loyalty. They refuse and Scar kills them instead. I assume he tells mufasa some sort of lie and mufasa believes it.
We see the guard in the clip, playing some sort of sport. Bungaa and kion get too close to what is known as the outlands. Bungaa is captured by hyenas and Kion doesn't want to go into it knowing he'd get into trouble.
He lets out a majestic roar and saves bungaa.
While we don't know if kovu is in, kiaras actress on twitter gave a 'can't say at the moment *wink wink nudge nudge* leading me to believe kovu is in but they're still waiting to reveal him.
Is there anything i've missed?