Complex sentence: Nelkin felt the series managed to explore of Mitsuki comparing that while he initially expressed not emotions like "Naruto" character Sai, that "is growing as a person as he spends time with his comrades".
Split into simpler sentences: Nelkin felt the series managed to explore more of Mitsuki, and compared him to "Naruto" character Sai since he initially did not express emotions. Nelkin further described Mitsuki as a "character that is growing as a person as he spends time with his comrades".