After having an emotional confrontation with his best friend, Light was left gravely wounded. The injuries were life threatening, his brother having to quickly amputate the mangled limbs during the operation. He couldn't keep him there for long, the two being hunted in their own home town. His brother, Spark, ran with his Light's fading body to a nearby abandoned town, using what was left in a medical facility to stabilize him.
Losing so much blood left Light's body weak and even with a transfusion set up, Light passed away while Spark was in the middle of one. He was able to shock him enough to bring him back, but Light remained in a critical state for the next month of his coma. Spark spent that time ding whatever he could to help him. He gave his scarf to Light to keep him warm despite the Heilolisk constantly walking out in the cold for supplies. Despite his drained, exhausted body slowing him down, Spark was able to keep them both alive through the worst of it.
Using the resources of the area, Spark sets up a lab underground in a market's basement where supplies were once stored. Light remained immobilized for a few weeks, unable to grasp his reality with no sight and missing limbs. Spark created prosthetics for Light but he struggled to get used to them. The next few months centered around Spark using his knowledge of medical and technology that he spent his entire life studying to help his brother out.
Even with the new limbs, Light still lacked the ability to move like he used to in them. Spark proposed a new surgery to remove the rest of his healthy limbs to train Light's body with those. More months pass, Light now heavily modified with technology. His parts require a lot of maintenance but they are easy to disassemble. Spark's work pays off however, his once crippled brother now moving on his own again.
Living in isolation has done a toll on Light. A year of surgery and no one to communicate left him very stoic, but the two brothers are now closer than they ever have been. They remain cautious, and scare away anyone who sticks around the abandoned settlement for too long. Eventually, Light masters his body. While he is more powerful than a normal Helioptile, his body has limits.