You could say “he is a motivated person.”  Proactive simply means that you are taking an action before an expected event or outcome. If I understand correctly, you are talking about someone who studies on their own without prompt. I would not use proactive. An example of the use of “proactive “ would be: “The city is taking proactive measures against crime.” Or “Emergency training is a proactive step taken for security measures.”  You could also say they are “a go-getter”

Hello Phil. Actually I just want to describe a personality where someone works with little supervision / instructions, and he / she can proactively work. In this case, which word is proper?

Do you mean "motivated" or "motivating"?

Hello Michael, thanks for your detailed explanation.

Hello Phil, thanks for your prompt reply.