Inspired by this picture: http://wonderfulworldofmoi.tumblr.com/post/128722313924/miraculous-ladybug-secret-identities-shh-i
Chat convinces LB to trade journals (or one journal that they both write in) so that they can be more like friends but still keep things secret. LB grudgingly accepts and keeps her notes to things like “The sun felt really nice on my skin today.” “I was inspired by the skyline of Paris while out on patrol. I got so much done when I got home!” And Chat sends her little romantic drabbles (that are totally about the two of them but he uses aliases and she remains oblivious) in hopes that she’ll be more receptive to him when he asks her out. LB never says anything to Chat, but she gets totally excited over receiving his drabbles because she just loves them and pictures if it was her and Adrien.
This leaves room for multi reveal options, either hand-writing recognition, story mention, one of them having the journal with their civilian self etc.
Submitted by @violetgalval