Naruto's character would develop in a rather dark direction. He'd become more secretive as he doesn't want his friends to know about his use of the dark side. His sense of humor might disappear. He'd be constantly fighting the urge to give in to the more extreme aspects of the dark side, like aggression and a thirst for power. This would also make him more self - reliant in a negative way, as he may start to think that only the dark side can help him in difficult situations, rather than relying on his friends and the power of friendship like he used to.
His character would become more complex. He might start as a naive user of the dark side, thinking he can control it. But as time goes on, it would make him more brooding and solitary. His once bright and optimistic view of the world would be clouded.
If Naruto uses the dark side of the force, his character development would be quite a journey. Initially, he may gain a sense of power and confidence that he didn't have before. However, as he delves deeper, his values would start to shift. He could become more ruthless in achieving his goals. His relationships with his friends, like Sasuke and Sakura, would be severely tested. They might try to bring him back from the dark side, which would lead to internal struggles within Naruto. Eventually, he would either succumb completely to the dark side or find a way to redeem himself, and either way, his character would be forever changed from the innocent boy he once was.