Hello Gonta, welcome to UE!
It's a little difficult to say, without more context; but uncles are popularly supposed to be genial, generous people.
So by saying "Uncle X is here!", he is humorously presenting himself in a kindly, benign, generous, indeed avuncular light.
It is possibly also a little self-deprecatory; uncles and aunts tend to be regarded as harmlessly asexual, by their nephews and nieces. Perhaps that's what the character wishes to imply, in your context: generous, but without "ulterior motives".
All the best,
Student or Learner