Man been poking at this a little and like shit's starting to blur as this feels legit kinda schizo for how it'll be done.
He'd always call the main character Lamy.
Wake up? He's floating there.
Come out of the shower? Oh he's sitting on the sink where there are towels you didn't (think that you) put out. After all he's there to help Lamy.
Be at work and he's floating there in the corner of your vision just enough that you can tell he's there but not fully see him (or are you imagining this).
Then you start getting behaviors getting weird like checking the roots of your hair to make sure they're not turning blue as he insists you're just going to become Lamy so why not accept it?
Like if this wasn't the Lamy TF thread I'd kinda consider it being that the MC is legitimately going insane and Daifuki's a manifestation of it.
Or have the MC end up in a 'mental institution staffed by women who look very similar and have blue hair and elf ears (or do they?) And they wait until their new sister finally calms down and comes to her senses