The only thing that comes to mind when I hear little soul is it refers to a baby's soul. If I listened to the song that would probably give me more information but if someone says in English little soul I think of a young soul like a baby that has died. But I listened to the song and there were no babies and it was a very beautiful song. What I will say is that in Spanish and italian using the word little adds a cuteness to someone or a thing. So if I say pequeñito it means little thing or tiny but is a cute way of say pequeño. And adds a cuteness or softness to the thing. So maybe in this case the mean a soft, caring, loving and soul.

Dear Ben, Thank you for your letter. Sorry I have not written in so long due to me being really busy. I am glad to hear that you started learning Japanese. I am learning English in my school. I enjoy learning English and I think this language will be useful to me. I understand that this is a very good opportunity for me and I am trying to improve my knowledge of English. I am thinking of choosing English in my future career because I am good at it. All in all, learning a foreign language is a very important thing and I am glad that you are pursuing it with starting to learn Japanese. Anyway, I have to go now because my mum asked me to help her with washing up. Coming back soon. Love, (You put your name here).