I love supporting characters. In a series like THE REVENGE GAME, it's very easy for the lead characters (Wes, Brandon, Corey, and Greg) to become mired in a lot of drama, which is great! However, I think every book needs a fleet of supporting characters who can add levity to the situation and lighten the tone from time to time.
When I first wrote, THE REVENGE GAME, Madeline, Kirby, Mo, Jack, and Holly were people who came about and were there to be there. I knew Madeline lived across the street from Corey, but then I wrote her first "scene". Wow! I fell in love with her snooty attitude. She reminds me of Nellie Oleson! Madeline wasn't in my initial outlines for THE REVENGE GAME, but she was so much fun to write, I couldn't not include her in DECEIT as well.
Mo Gross came about when I wrote the first story bible for the trilogy. I wanted Brandon to be the baby of a large family and Mo was the eldest child. Well, there was nothing remarkable about Mo until I started writing THE REVENGE GAME. As I started exploring the characters' families, I picked Mo, made him an irresistible jock jerk, and quickly paired him with another character (if you've read THE REVENGE GAME, you know who!). It all made so much sense, but I didn't plan it. Having a slew of supporting characters to draw on allows you space to expand the world of the novel.
With Jack Webber, he was only supposed to appear in the graduation scene. He's Wes's old friend, but they drifted apart during high school. Jack was the "opposite" of Brandon, Corey, and Wes, but he came from the same world. When I paired Jack with another supporting character, it made sense. We see a lot more of Jack in DECEIT, but that's all I can say...for now...
Even though supporting characters may not be in your initial outline or pitch, they can add a dimension of reality to your world. It's a good thing!