The fact that Kai had entered Ash's office was no coincidence. When Raven had made the request to meet with her brother, she assumed it wasn't because he wanted to hash out his feelings or challenge him to some sort of honorable fight.
It was clear he wanted to talk to Gary because of his position. There were multiple times when Raven had mentioned his current position, and with nowhere else to go, he had to turn to the Howlers.
So, Amy had decided to call Kai instead. She knew exactly how her brother would react, especially if he learned that Raven had met her behind his back.
Amy had been in contact with Kai several times because it had simply become easier for the two of them.
Constantly, Amy would ask Gary for a few things, such as apartment contracts, to open up bank accounts, and so on. Each time, Gary would have to contact Kai.
In the end, it was just easier for Amy to contact Kai directly, and because of this, the two of them had developed a closer relationship.