How do you explain a character who isn't real until she's not? Stick exists in the maybeland, the uncertain spaces of reality. She is both an artifact and an anomaly of them. There is no origin, no background story, no lore. There's no "why" to Stick. She just happens. Like water flowing through the cracks of reality. The most real thing about her is her closeness to you. Where you are, Stick could happen. This relationship is axiomatic, not emotional. Married in a mathematical sense.
Stick doesn't arrive or leave, she's a possibility that could happen anywhere you're not looking. What is behind a closed door? You don't know until you open it. It could be Stick is waiting for you on the other side. Gaps of attention or knowledge are Stick's entrance. You might think you're alone, in an empty room, but what's behind you? Under the bed? In the closet? She doesn't arrive in that sense, because that implies a journey from one point to another. Stick is either there or not, becomes actual in a place she could be. And if she's not there, she's not not there.