Mowgli is shown to be adventurous and rebellious. At the same time, he is fairly naive about the dangers of the jungle. He is shown to love having fun, and is trusting and friendly. He does not sport a healthy appetite, however at times he is shown to have Baloo's lazy, laid-back nature.
Mowgli does not gain weight in either of the movies he's in. However he does gain weight after continuously eating a unique fruit called Caloberries in a fanfic by comicfan67 on deviantart.com. The chapters to this fanfic can be found here: http://comicfan67.deviantart.com/gallery/?q=jungle
Mowgli also gained weight along with Ranjan and Baloo in another fanfic written by comicfan67, which also coensides with an illustration by mothman64. Both image and fanfic can be foun here: http://mothman64.deviantart.com/art/Jungle-Book-Weight-Gain-302871695