Imp defenders are only found in Puro-Puro and their function is to attempt to release one of your jarred implings whenever you're near them. If successful, your jar is also knocked a few steps away. Their success rate relies on your Thieving level, however, carrying Imp repellent will also lessen their chances of releasing an impling.
Imp defenders will shout "Be free!" when they succeed in releasing one of your implings and will only release the lowest-levelled impling you are carrying on every successful attempt; therefore, if you are hunting higher-level implings, it is advised you carry around about 2 lower-levelled implings, so that if an imp defender manages to release one of your implings, it isn't a priority one. If the released impling is higher levelled than nature, it will not respawn.