Notes:

This is the nearest possible translation of this post
I was requested by IJK_Principle to translate this about two weeks ago! I didn't noticed I was requested to do it until today! My bad!
The text doesn't seem to make much sense, not even from the Japanese POV. Serena is most probably afraid of Ash treating her as a sister, hence the "セレナ..おねぇちゃん?/Serena… Oneechan?". Ash literally just tells Korrina that "he's likes training/he's also a trainer".