Complex sentence: Subaru's sense of duty towards Athena is not his best asset, despite having prodigious natural gifts, and he harbors an intense rivalry towards Koga, constantly seeking his flaws to defeat him.
Split into simpler sentences: Subaru's sense of duty towards Athena is not his best asset, despite having prodigious natural gifts. Subaru harbors an intense rivalry with Koga, and seeks his flaws to defeat him.