Cail is actually quite the interesting character in my verse.
You see, not only do we have the fairly classic fanon of him knowing Wes, there's also the fun part where he knows Nascour as well. It started out fairly innocent too - they were both rebellious teens looking for a fun night out in Pyrite Town, with Cail living in the area to this day and Nascour coming over on Metang/Metagross after escaping from his family home near Phenac City. The two boys would chat and even engage in some experimental flirting and fooling around. To this day, Nascour will often go and say hi to Cail when he's in or near Pyrite.
What Cail never knew, however, is that Nascour has since become one of Cipher's leaders. It's even possible that Cail knows things like Miror B. being secretly backed by more powerful people from Nascour, being fed random tidbits of information that Cipher wants out there (no, of course they want nobody to find out about Evice and Greevil, but Nascour is mentioned pretty openly by Cipher people; he's the boss that people are supposed to learn about as both a threat and misdirection).
Cail isn't stupid. He would've eventually noticed that other criminals run and hide whenever Nascour is in town - if aura manipulation didn't keep him nice and comfortable in the realm of guilelessness. For the longest time, he remained entirely unaware that he was "friends" with a very dangerous person and invited in evil that was far beyond the mere pranks and shenanigans he gets up to. Nascour saw no benefit in revealing the truth, thinking that one day a good opportunity would surely present itself, and he would use trust rather than fear to deal with Cail until the time comes.
The details of what happened between Cail and Wes aren't set in stone. But they were close. And Cail had and still has strong feelings for Wes. Once he found out about that, Nascour rejoiced - in fact, he was much more delighted by Cail's attachment to Wes than anything pertaining to himself. Because such emotions could be twisted and then used. Turning love and affection into obsession and jealousy, an easy task with Rui in the picture.
It may be that Cail accidentally or even knowingly leaked information to Cipher, due to promises that were made to him. It may be that he learned who Nascour really was during this time and cooperated only out of fear. It may be that he risked his own safety and well-being by trying to mislead Cipher and/or warn Wes. None of that is set in stone either.
But this is basically what's going on with Cail and what past ficlets featuring him have been referencing.
