Though not much about Marni's life apart from her associations with Rotti, Nathan and Mag are revealed, when Mag shows Shilo the hologram projection of Marni, Marni is singing. It is possible that, like Mag, Marni was a singer, or at least had an interest in it.
Marni and Nathan eloped after Marni left Rotti, and it is safe to assume that their relationship was a happy one, for however long it lasted. Nathan was devastated after Marni's death, and carried the grief for seventeen years.
Though she left him at the altar, it is generally believed that Marni was in love with Rotti prior to her meeting Nathan and falling in love with him.
Though their friendship was never explored indepth, Mag and Marni were known to be best friends. Marni was the one who introduced Mag to Rotti when the pair was dating. During the flashback to Mag's surgery, Marni is seen standing by Mag's bedside.
"Chase the Morning"