Unique.
Not only does this word explain the circumstances behind both of their births, it also perfectly surmises their relationship.
On one side, you have the Master of Chaldea: Stuff. His recent birth came from unique circumstances where convenience had it that a boy with an earnest desire to live was found close to enough power to do almost anything.
The resulting existence could no longer be classified as a human, yet it didn't fit the requirements to be a servant. He couldn't be called a phantasmal either due to the nature of his existence. The classification that has been imposed on him is that of the Foremost evil of humanity: Beast IV:Retain, but even this isn't fully accurate, so as far as he cares, he is nothing but Stuff.
On the other side of this relationship stands a loli archer with charms to rival a succubus, Chloe Von Einzbern. Stuff knows very little about her, but could tell she was the product of some form of rejection from a host body. He never wanted to bring up her origins because it would most likely be depressing for everyone involved.
As for what she is, Chloe considers herself a normal servant, but everything screams that she hasn't become one through normal means, and that whichever world she comes from is flawed and possibly doomed to disappear.
In a way, Chloe and Stuff are very similar and are probably the closest life forms to one another. Freaks of nature, barely fitting their respective labels but not letting it get in the way of life. This proximity is most likely what kick-started their relationship into what it currently is.
What is their relationship? Putting it simple, they are kin, and they complete one another down to their very existence. Stuff was born with a tremendous amount of magical energy, one that he can't keep hold of if he doesn't want to explode into a world ending catastrophe. He needs to let it loose almost constantly, generally by seeping it into the world for him to keep himself stable. And Chloe's existence was unstable, she needs a regular supply of energy if she wanted to keep existing. This fact became even truer with her incarnation as a servant, in a world that wasn't hers.
All that to say, they be fucking. Like, a lot. Their main intent has never been to solve each other's problem. Their propension to have fun, along with their natural affinity made them fucking the only logical outcome of their meeting. The longer they hung out together, the more of that time would be spent fucking. It went to the point where Stuff's room had a permanent reality marble stopping time to allow for this disruption of his daily life. And that is at one such times that an unforeseen visit happened.
"It was a nice way to spend 24 hours." Chloe was the first to come out, bouncing around, leaking bits of... residual mana all over the floor of Stuff's room. She stretched a bit before looking behind her as the Master of Chaldea came out of his own den.
"We definitely over did it." Stuff's demeanor didn't change. He kept the same slow walking and absent minded behavior he always has.
Although this didn't stop him from catching Chloe when she jumped into his arms. "If only there was a reasonable adult who knows when to stop us." She nuzzled the Master's cheek.
"I've been looking for one my whole life." He kept walking, though there was a slight desire to turn back and have his way with a tanned succubus.
But this chapter is family friendly, and cockblocking was bound to happen. This happened under the form of a portal opening up, revealing the intruder of the day.
"This is where Chloe should be. We need to be as cautious as possible and-"
"Illya!" Chloe jumped away from her Master towards the trespasser.
