The equivalent of your typical RPGs MP (magic points), Eve is what allows you to use Plasmids. The blue bar below your health indicator shows how much the player has left. Eve is supplied through Eve hypos (hypodermic needles), which are injected into your arm.
Hypos can be obtained throughout Rapture, from slain enemies, some food or drink items (Tonics can affect this), and Vending Machines. The maximum amount of EVE that you have can be increased as upgrades at Gatherer's Gardens.
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