Lost In The Void (Voltron OC fanfic)

Ren was just a 14-year-old student at the Galaxy Garrison when he was chosen as a ride-along on the next big mission to work as medical support. After the rest of the Kerberos crew set off on a space mission with him, they all fall into the clutches of the Galra, an alien race infamous for the conquering of entire galaxies with ruthless might. As the others are taken away from Ren, he finds himself immersed in the struggle to keep his promise; to survive the excruciating treatment that offers little hope for escape.

Now 16 years old and trapped in the same prison, he thinks he is not going to be able to keep his promise anymore, but Shiro comes to Ren's rescue as he is bleeding out. As he is introduced to the paladins and their true purpose in staying in space, he realizes he must stay to help them achieve the goal of taking down the Galra. As he battles the trauma and flashbacks from his time as a prisoner, Ren's journey becomes an unforgettable fight of resilience against all odds.