Since I haven't read the 'tomodachi no imouto light novel', I can't be certain. But usually, the main character would be the one whose perspective the story is mainly told from, probably a male character if it follows the common light novel trope. And of course, the friend's sister would be a major character. There might also be other friends or family members who play important roles in the story.
The main characters in the 'tomodachi game light novel' include Yuuichi Katagiri. He is intelligent and resourceful, always trying to find a way out of the tricky situations in the games. Shiho Sawaragi is also a main character. She has her own set of values and beliefs which often come into conflict during the games. Another important character is Tenji Mikasa. His actions and decisions also play a significant role in how the story unfolds as he is also deeply involved in the friendship group and the games they are forced to play.
Yuuichi is definitely one of the most interesting. His intelligence and ability to manipulate situations make for great storylines. You can explore his motives and how far he's willing to go to win. Another is Tenji, his complex relationship with Yuuichi and his own moral code can be a great focus.
The title suggests that the son and his friend are key characters. The son is a part of the equation as it's his friend involved. There may also be a parental figure who is doing the entrusting. Without more information, it's hard to say who else might be a main character, but it could be someone from the son or his friend's social circle who gets drawn into the situation.