Jerome (Eugene) Morrow was a 'valid' who possessed what were considered to be optimum genetics. This gave him a systematic advantage in the working-world, making him eligible for any job he pleased. He was a former swimming star, but placing second in a race lead to him falling into a deep state of depression. As a final resort to rid himself of misery, he jumped in front of a moving car in a suicide attempt but was unsuccessful. From this point onward he was paralyzed from the waist down. Due to the horrific incident occurring overseas (the specific location was not clarified) no one knew about his new disability, and Jerome was happy to keep his failure secret. He distanced himself from society and lived as a hermit where he could quietly be disappointed with himself.
So as Jerome had ‘perfect’ genes and had given up on his own life, he was the perfect person for Vincent to ‘become’. He didn’t want to use his life and as such was happy for Vincent to able to use it. Even though Jerome now had a purpose for his life in helping Vincent become him, he was still depressed about his failure and eventually did kill himself at the end of the movie. He kills himself so that there is no confusion about the two Jerome's.
Jerome was similar to Vincent in his basic appearance. He was of a similar age and also had dark hair. While they looked similar, Jerome was a fair bit taller, and they both looked very different from each other. But as Jerome had been hiding from society, no one had seen him in a while and Vincent could be made to look enough like him to pass.
Morrow ultimately ended his life at the beginning of the film by burning in a garbage incinerator. This was presented rather confusingly because of the constant flashes to Vincent going to space. Morrow wore a medal and could be seen through flames proving this.