I feel bad for keeping my rat alone in cage?

I don't think there's much you can do. Try this if you have not already tried it: divide jemains cage in half with a fence, and put jemain on one side and kenan and his cage mate in the other. Let them sniff eachother, don't mask their scent! If jemain gets aggressive spritz him with a spray bottle. Eventually you should be able to lift the barrier, and if jemain hurts them remove them and spray jemain again.