In this isolated sentence, the use of 'but' seems strange.
Perhaps the speaker was responding to the question: "How's Miki?"
The first clause of her answer has the idea that she only knows a little about Miki.
She continues with a comment on Miki even though she has little information about her.
In the common utterance "I'm sorry, but I can't come," there is also no contrast between the two clauses. Perhaps the contrast is between my inability to come and my desire to come. "I'm sorry" implies that I want to come.