Complex sentence: After several rounds of the tournament, Kokoro meets Helena who, unbeknownst to Kokoro, is her half-sister; however, Helena seems to recognize Kokoro and mentions Miyako before their fight.
Split into simpler sentences: After several rounds of the tournament, Kokoro meets Helena who, unbeknownst to Kokoro, is her half-sister. However, Helena seems to recognize Kokoro and mentions Miyako before their fight.