I loved the series and watched it four times.
The first male protagonist(Park Jae-Chan) was very good because he really seemed like a computer man and how he changed emotionally thanks to the meeting of the second protagonist.
The second protagonis (Park Seoham), however, showed me the change in the boy's maturity.
When he fell in love and realized he was truly in love, he transformed her into an adult.
It's very difficult to make the audience understand these changes so the two actors were excellent.