If you don't know what I'm talking about, contrast 君はレンくんね with 君がレンくんね.
In the first sentence, the topic (as opposed to the subject) is "you". The subject, however, is implied – thus there is no emphasis on it. To use a different example, お前は少佐なのか would mean, "Are you a major [regardless of whether other people are also majors]?"
In the second sentence, "you" is explicitly defined as the subject, thus emphasizing it. To use my other example, お前が少佐なのか would mean, "Are you the major [as opposed to anyone else being the major]?"
Granted, in Len-kun's case, both would probably be translated the same, but there is a difference in emphasis. I'm surprised you haven't picked that up yet.
seabook said: The later example with shousa illustrates the difference between an indefinite and definite article; I don't see how it relates to the phrase at hand.
There are no articles in Japanese. In that example there is a difference in meaning that can be represented with articles in the translation, but in Japanese it corresponds to the difference between は and が (topic and subject).
Just like LaC said, I was just using articles in an attempt to illustrate the difference in meaning. That might not have been the best example ever, but it's hard to think of something like this on the fly. I'd still like one more go at clarifying my point.
Basically, I'm saying that the corresponding statement 撲がレンくん would translate as "I am Len-kun" with the emphasis on "I" (no one else is Len-kun) whereas the statement 僕はレンくん would translate as "I am Len-kun" with the emphasis on "Len-kun" (I am not anyone else).
