A person with whom one is in a state of mutual hostility or animosity

शत्रु (Shatru)

English Usage: They were sworn enemies since their childhood, always competing against each other.

Hindi Usage: वे बचपन से शत्रु थे, हमेशा एक-दूसरे के खिलाफ प्रतिस्पर्धा करते थे।