Frustration was an emotion with which Chimera was quite familiar, and it was not pleased by the familiarity. Its current source of frustration was trying to solve a problem with no solution; there was no way to tell whether or not a human was a Gate without possessing a Phantom's particular senses, and there was simply no way to screen the population of a city as huge as Tokyo for potential Gates and then go about handling their inner Phantoms one by one.
Simply put, Chimera was fairly sure that there was little or nothing Soma could do to stop the inevitable wave of Phantoms that would wreak havoc on the city. If they were lucky, the horde wouldn't all mature at once, but Chimera didn't like to count on luck. The upside, of course, was that once the Phantoms were broken free of their hosts, they matured properly and were delicious, and Chimera was looking forward to having something to hunt again. Getting mana from Soma was enjoyable, to be sure, but not quite the adrenaline-pounding pleasure of a chase and a kill.
Even the prospect of a significant increase in prey did nothing to blunt Chimera's frustration, though; he'd tried to do something human and it had backfired spectacularly. He wasn't sure that he felt the same thing Soma did, but knowing that he enjoyed having Soma around and would miss him if he were gone was surely close enough. He'd tried to return Soma's feelings, only to have Soma accuse him of breaking his heart and obsess on the problem of the Gates.
Chimera had tried to use actions instead of words to convey affection, and Soma had thrown that back in his face. He stretched out on the bed that served no purpose if Soma wasn't there to sleep on it and stared at the familiar ceiling with a faint sense of resentment. Chimera closed its eyes and sank into what passed for its Underworld, searching the tangle keeping it trapped once again.
The threads holding Chimera in place were constant; the shape of a tangle blocking the only exit out of Kosuke's Underworld didn't change, though the threads had enough give to allow Chimera to slip part of the way out of Kosuke's body. It couldn't work its way entirely free through the narrow gaps, but it could get most of its essence past the barrier while the threads tangled in the rest. It did so now, looking at the tangle from as far outside of it as possible for the first time, and noticed something new. The silver thread it had severed months ago in its first attempt to break free of Kosuke's body and the Beast Driver hadn't vanished; it was wound through the base of the tangle, where Chimera hadn't been able to see it from inside the remains of Kosuke's underworld.
Getting closer to the silver thread required some odd mental contortions, but Chimera was nothing if not flexible. It angled its perceptions down toward the silver thread, keeping an absent eye on Kosuke's body. The severed ends of the thread hung, dangling in a way that seemed intrinsically wrong. One end led to a root grounded in Kosuke's Underworld, as it should have been. The other end spooled off into the tangle. Chimera tried to follow it, nearly losing it in the twists and turns that were less familiar from this direction, aware of and ignoring the growing ache that came with the unnatural manipulations of its spirit.
The end of the silver thread branched, when Chimera finally found it, splitting off into countless threads running deeper into the tangle. Chimera reached just a little too far, and the sense of a tether snapped it back inside Kosuke's Underworld, but it knew where the silver thread had separated out. The silver thread wasn't lost in the tether – it was the tether. The impassable thicket binding Chimera inside Kosuke's body was a trap composed of Kosuke's soul.
Chimera surfaced with a sense of uncertainty, opening its borrowed eyes – and there was no doubt now that they were borrowed, if Kosuke's soul was trapped along with Chimera – to see nothing resembling any kind of answer in the ceiling overhead.
Taking a deep breath, Chimera placed itself in Kosuke's Underworld again. "Nitoh Kosuke?" it said, but there was no answer, just as there had been no answer when Chimera had first found itself trapped. It lay a hand on the spun-thread barrier of Kosuke's soul and tried again, to no avail.
The artificial light was almost painful when Chimera finally climbed out of Kosuke's Underworld, having given up trying to get anything resembling a response from its former host. It wondered briefly if knowing that Kosuke wasn't gone should change how it felt about escaping Kosuke's body, but it was a moot point; even if Chimera could give Kosuke his body back, it wasn't sure it wanted to.