The end of the episode brings to light of a missing back up crew member. Nanba Mutta takes count that there are only five of them present, and a 6th is needed to have a complete compliment. In fact, the chief later reveals the missing position as the back-up crew captain.
The chief mentions that the position isn’t entirely confirmed because of an initial rejection from the prospective crew captain. The screenshot below shows the foreshadowing scene at the end of the episode.
From the way the chief explains the situation and how the episode ends on this scene makes me think it’s someone familiar and not an entirely new character.
Just prior to this episode, Nanba Hibito has just conquered his PSTD and is recovering from his debilitating condition to get back on track for active duty. Now one might immediately think ‘Oh it’s got to be Hibito!’ but the circumstances are way too soon. Any normal administration wouldn’t put someone who has JUST ‘recovered’ into a position where it requires him to be beyond 100%. So in my books, the mysterious captain cannot be Nanba Hibito.
So who could it possibly be? If you can recall, there was someone else that trained along side Nanba Mutta during NEEMO training for the same chance to get selected into space. That person was Nanba Mutta’s best friend Makabe Kenji. During the last bit of that arc, the chief selected Nanba Mutta over Kenji but in no way denied Kenji. In fact, the chief put it in words as if he had other plans for Kenji. That arc ended with Kenji and another familiar Japanese astronaut going off to start some other training that was not disclosed to the viewers.
Now this doesn’t mean that the mysterious captain could be BFF Kenji, but I feel he is a likely candidate for that position. We will just have to wait and see what happens in the next episode.