Genis is a child prodigy. He is Lloyd's best friend and miles ahead in intellect of everyone around him. He is also the youngest member of the party. Because of his age, we are willing to let men and women audition for this role. While usually very mature for his age, he is still a child and can sometimes act as such. He lives with his sister who has been like a mother to him.

Here is a reference scene. It's near the end of the game and provides some spoilers. But the scene by itself doesn't reveal too much of the plot.
https://www.youtube.com/watch?v=F_tRFPU5VcE