I think the best way to establish these rivals is to have them at least introduced early on so the readers know they exist. Every time a character is introduced like this, it just feels like they were poofed into existence only to serve a well worn point of romantic conflict.
I mean, if this girl is meant to have a brief appearance and quickly crash and burn to serve as filler for a few chapters, i guess thats fine, but its still a very tired trope. As long as she's not a permanent rival.