Once Clyde steps back, the blue-haired young man in front of you notices your presence, turning and declaring that you must be a challenger.  He then welcomes you to the Gym, and introduces himself- this is Cheren.  He then points out that he and the two others need to prepare for your Gym experience.  Cheren calls for the two beside him to take their places, and tells you that if you can defeat them, he’ll be waiting to battle with you.

Cheren heads off to the far end of the yard, leaving you to make your way across it.

If you want, you can avoid both Gym trainer battles here by going to either the extreme left or right edge of the yard and running up it- but there’s very little point.  The trainers are both less imposing than the leader (as is virtually always the case in Gyms), and if you defeat the Gym Leader in a Gym, all of the trainers there become inactive- effectively robbing yourself of money and experience.

The first of the two challengers explains that Cheren saw potential in him and brought him into the Gym.

Battle: Youngster Pedro (144 Poke)
Opponent: Patrat Lv 9, Lillipup Lv 9

The girl, meanwhile, is eager to show off what Cheren taught her.

Battle: Lass Serena (144 Poke)
Opponent: Lillipup Lv 9, Patrat Lv 9

With those two defeated, make sure your team is in fighting trim (heading back out to the Pokemon Center if you find you need to) and then move up to Cheren’s platform to challenge him.  When you talk to him he says that, just like how this is your first Gym Challenge, this is also his first time battling as a Gym Leader.  He encourages you to do your best, apparently not caring too much who wins and who loses the battle.

Battle: Gym Leader Cheren (1560 Poke)
Opponent: Patrat Lv 11, Lillipup Lv 13

The only outright difficult part of battling Cheren is that he carries a Potion, but it’s easy to get him to waste it on his Patrat.  His Lillipup does know Bite, which can be a bit threatening if your starter is on the frail side, but for the most part you should be safe in this battle.