dr jekyll and mr utterson both have hidden pasts. What are they ?

I'm not sure what you mean by hidden. Utterson has known Jekyll for a long time. he recognizes the changes and strange occurrences of Jekyll and Hyde, and resolves to further investigate the relationship between the two men. Utterson is a middle-aged lawyer and respected by most people. Aside from Jekyll's creepy experiments which we find out about later, Jekyll seems to be mild mannered. He used to attend social engagements but dropped out after awhile.