The second series i watched, who didn't have him as a principle character at all was Sunao ni Narenakute. It's a Japanese drama about this girl who falls in love with a guy. Jaejoong's character kind of falls for her too and things get complicated, like always. But the difference between Cha Mu Won and Park Seonsu( The Doctor) is huge. Firstly, he seems more of a innocent young man in the Japanese drama while he goes to the opposite in the Korean one. At the same time, as Mu Won, he does show signs of childishness so it kind of clicks. It's good to see versatile roles but it also gets a lot of critics. But I, personally, admire him entirely. As a person, he got through a lot, from being adopted to working hard for his dream. He does have the right to brag about what he became because he worth it.
Overall, he's on the roll with acting and will always be a good singer. That's his job. His place is on the stage and even behind the lights. That's one amazing guy, in my view. And, his future wife will be one lucky woman. :)