Am I missing something? Why would they need body armor?
Presumably the body armor would protect from love bullets/ammo - the only times in Chapter 0 the Cupids were hit were when body armor was rendered useless (Kanna was shot in the head and there's not much you can do with a grenade at your feet).
In general it looks like more of a personal preference for the Cupids. Chiyo is the least protected + casually dressed in just a flight jacket but is clearly the most competent fighter of the gang, so it could speak to her confidence in winning bouts with other Cupids without needing any protection.
knee pads/gloves aside, i was thinking koharu's body armor was related to how she died, but maybe that's just a coincidence.
really enjoying this series so far!