Edited sentence: Lacie was a very important person to Glen.She's aparently dead(since in a flashback it's shown Jack and Alice visiting her grave, the same one that Oz and Gil found at the start of the story) because she was a sacrifice made to the intention/will of abyss.She(for the few pictures shown of her) looks a lot like Alice, that maybe be releated to the fact that 'Lacie' is an anagram of 'Alice'.The melody that Glen composed was named after her