Ostensibly it's Pinbacker, the captain of the Icarus I who appears to have had a religious experience with the Sun. Following this, he becomes insane (or enlightened) and sabotages the original Icarus I, killing everyone inside directly or indirectly.
Seven years later after the original failed mission, when Icarus II docks with Icarus I, Pinbacker seems to have sabotaged the airlock and somehow found his way onto the Icarus II where he sabotages the ship in the same way (overheating the mainframe) and continues being a violent psychopath.
However, it's unclear whether Pinbacker actually existed or was merely a figment of Capa's imagination after Capa viewed the video of the insane Pinbacker on Icarus I. With both interpretations there appear to be several issues.
Problems if he's real
- Pinbacker survives for 7 years without shielding on the Icarus I. He might have had sufficient food, water and air since his other crew-members were dead.
- He appears to have sustained very serious burns all over his body yet survives for 7 years.
- There's thick dust all over the Icarus I, suggesting there has been little movement for a long time.
- He manages to find his way aboard the Icarus II somehow without anyone noticing
- The computer (named the Icarus) doesn't alert the crew about the new crew member who suddenly appeared
- He doesn't have a distinct appearance. When he is show on-screen it's momentarily, unclear or just a silhouette.
- He manages to magically teleport from the exploding Icarus II to the payload to further thwart Capa's attempts to detonate it.
- Nobody else appears to see him (clearly) besides Capa
Problems if he's imaginary
- He murders Corazon directly and pretty unambiguously
- He wounds Capa directly, leaving with a large visible gash across his torso
- He fights Cassie and Capa on the payload
- Capa is locked in the airlock and unable to leave due to the manual override that seems to have been pulled by Pinbacker
- The computer notes there is a 5th person onboard draining oxygen reserves
I'm still left rather confused after the movie. Is there any clear indication in-universe or from the directors/producers/etc about the intended interpretation?