Hola!
After a while of asking myself how to deal with those sections, decided that if a character/twain/etc... appears in one of the books, no need to put that he/it was mentionned in the same book.
As to "appear" : to me it means we see the character into action, rather than have someone else tell his story.
Exemple : Raymond Hartmann is mentionned in The Long War but we don't actually "see" him even if we know he's not far from our characters.