Complex sentence: Although she is very independent, her detachment often makes her feel alone, as seen in "Marceline's Closet"; while Jake and Finn are hiding in her closet, she expresses most of her feeling via the writings of her 500 year-old diary.
Split into simpler sentences: Although she is very independent, she often feels alone, as depicted in "Marceline's Closet". While Jake and Finn are hiding in her closet, she expresses most of her feeling via the writings of her 500 year-old diary.