Complex sentence: After having second thoughts, Nana encourages herself and decides to make team with Nyu, after all, viewing the teamwork as a competition test to prove Lucy that she's a more capable Diclonius.
Split into simpler sentences: After having second thoughts, Nana encourages herself and decides to make team with Nyū. After all, Nana is viewing the teamwork as a competition test to prove to Lucy that she's (Nana's) a more capable Diclonius.