Survival of the Innocent (A Voltron fanfic) (Book 1 of Fates Entwined)

One deca-phoeb, a Galra female by the name of Krolia kept the Galra from finding the Blue Lion. However, her ship received damages and she crashed. A human found her, badly injured, and took care of her. With time, this human gained Krolia's trust and her affection. A baby boy was born to them and they were happy. They though nothing could ruin their happiness; they were wrong.

The Galra picked up the signals of the Blue Lion and returned to Earth. She left to protect her family and the Lion but her husband followed her. He helped, but died. The Lion was safe but Krolia had lost her lover and her little child had lost a father. She took him with her, in the hope of keeping him safe and protect the Lion, but they were captured by the Galra. Krolia was forced to fight for their lives in the arena, but lost her own eventually. Her child grew up, learning the Galra customs and fighting styles, fighting - no, surviving - in the arena. He had privileges, allowing him to wander the ship they were all held on. He befriended the other gladiators and taught them how to survive, even the Champion.

Together, the Champion and the child escaped, headed to Earth, in hopes to protect what had cost the child so much and could change the fate of the entire Universe.

A Voltron AU where Keith grew up in the arena and met Shiro there. Cover art and the original show is not mine but my take is.