Complex sentence: The agency found success in both Korea and Japan with its first "idol" singer Se7en in 2003, who became the company's first artist to attempt to cross over into the US music scene, though his US debut failed to gain momentum.
Split into simpler sentences: The agency found success in both Korea and Japan with its first "idol" singer Se7en in 2003. Due to his success, Seven became the company's first artist to attempt to cross over into the US music scene, though his US debut failed to gain momentum.