Johnny is a engram construct inside a prototype Arasaka Relic.

As a Construct nobody except for you can see and hear him.

Johnny cannot interact with the environment.

Because it's a prototype, it was designed to be plugged into a dead host body so the construct could be brought back to life in a new body.

But Arasaka never tested it on a live host. Aka you.

Time is running out and you are dying thanks to him.

Throughout your journey together, he can't help but feel something towards you