rival (n.)
A rival is someone competing with you for the same thing. The word comes from the Latin rivalis, from rivus, a stream.12
Rivales were the people who took water from the same watercourse. It was a legal relationship before it was a hostile one: Roman law had provisions for what to do when two of them disagreed.12
The water fell away as the word moved through French into English in the later sixteenth century. The competition stayed, mostly over love or honour rather than irrigation.12
The shape of the old sense is still there. A rival is an opponent who draws on the same supply you do.12
He was her closest rival.