Scout

An extremely unique plasmid, this allows you to basically turn into a ghost at will. In "ghost" form you're invisible, but you can still use attack plasmids (no guns, though). This means that you can scout ahead and zap unsuspecting splicers standing in a pool of water, or temporarily deactivate pesky turrets without threat of attack. Your physical body will remain in the same place, though, and you'll have to return to it to progress.