Lavi + Tyki = Laki = Lucky - In japanese Rakki means Lucky.
The term "Lucky" is used also because Tyki and Lavi need luck to be able to be united due to this situation, of being discovered it would be a big problem.
Lavi is bookman and exorcist. Tyki is Noah and he have a life as human. For this both live through a double life.
And as both have created a friendly bow with the human beings in spite of the fact that they must not have any deal with these.
It is necessary to add that Tyki is the noah of the pleasure and Lavi cannot have feelings for being a future Bookman, so nothing better that to feel pleasure with an expert in the matter XDD.
Hoshino has put subliminal Lucky in the manga.
The proofs are these:
*Tyki and Lavi fight in Edo but on good having seen it, Tyki only seems to play with Lavi, never he are obvious by the intentions of attacking him seriously.
*Something that surely we we all will have seen, in them fight inside the ark, when Tyki Noah wakes his side up and attacks Lavi, this one licks his blood. It gives us to understand many things XD.
*And now something that probably even they haven't noticed for those that they read the manga:
Tyki and Lavi changed earrings in the new part of the manga, that is to say the new chapters. I will not give them any more explanations, better simply go on to this image: --> Sorry. In Spanish The image says [From left to right]: "The earrings that Tyki was using to the beginning were small and round." - "The earrings that Lavi was using to the beginning were of hoop and small." - "Transfer of earrings" - "Later Tyki uses Lavi's earrings. Of hoop and small." - "Later Lavi continues using them of hoop but bigger. [Since theirs they were given to Tyki]"
And it is all. I wait that, as to me, you like this perfect couple and that you share the motives that you have in order that you like it.