Qasi is about as important to Mai as Mai is to Mai, if that makes any sense at all... and to those monsters among us it makes very little sense.
Qasi first entered Mai's life as a ghoul of her sire, Winston Adonis. He had followed Winston's poetry and Winston had followed Qasi's intellect. He was ghouled around the same time Mai was embraced, so the two of them found they had much in common as they learned the ways of the night. Mai became incredibly close with him, thinking of him as a "brother".
Now of course he is much more. Mai's most prominent memory of him (aside from the death grip fear had on her when he was missing recently) was him being the one who helped her away from the Malkavians all those years ago. When her sire could not help, and the prince would not help, a mere human did.
It was unquestionable that Winston would allow him to leave for Detroit with Mai. It made both Qasi and Mai happier to be permitted to remain together.
Recently Qasi was taken for some nights into the hands of those who held Mai captive and she found exactly how difficult it was to find and rescue a loved one. She feared he had died and she could feel herself changing... her voice getting lower, her looks getting more haggard, her humour gone, her patience shredded.
Thankfully, he was returned to her, albeit having had a clear tampering with of his memories.... the upshot of which is yet to be seen. Mai now feels more of a tug to him than before, knowing now for sure that the moment he is gone forever is the moment she begins the slippery decline into beastliness. He is the reason she remains gentle most nights. He is the reason she remains kind, keeps up appearances, cares for the masquerade... it is for him and people like him.
It has been difficult for Mai to watch Alicia suffering after what happened to Jonathan Starr. Love is a very powerful tool for a vampire, and once it is gone... everything goes with.
Stryck Wiki