I forgot some of the details because it’s been a while since I’ve read side:red, but the basic gist goes something like this.
There is a facility under the control of the Gold King that is a hospital and research center on the surface. However, its patients are usually people who’ve gotten involved with supernatural incidents and injured as a result, and the research center is actually a gathering of Strains for the purposes of education and research. The research center is built like a school for Strains, and Strains living there are treated pretty decently and taught the necessary knowledge they need about their powers. That is the portion of the facility under the jurisdiction of the Gold King. Anna was in this facility because she had some ‘illness related to the brain’(which was just the cover-up excuse used, she’s really there because she’s a powerful Strain).
The head of the facility, Mizuchi Koushi, is a Gold Clansman who had his ‘gold’ aka ‘talent’ drawn out by the Gold King over ten years ago. His medical talent basically turned into healing powers, and he’s said to be able to regenerate even lost limbs.
However, rather than being overjoyed at getting his talents drawn out like this, Mizuchi began to become frustrated, because by drawing out the full extent of someone’s talent, the Gold King is basically removing all future potential from that person and saying that, ‘what you have now is the best you’ll ever have’. Mizuchi didn’t like the fact that his ‘fate’ was sealed like that. So he began to plot and crave for even greater power. He saw the Kingless Blue Clan as a chance for him to obtain exactly that - he’ll offer the Blue Clan a new King, an easily controllable, King-in-name-only sort of person that he can manipulate from the background, so he can basically take over the Blue Clan. To this end, he began conducting secret experiments on various powerful Strains held in the underground facility - all while hiding his activities from the Gold King and justifying his actions with “his research will uncover more information about the Dresden Slate - which is what the Gold King wanted”. The Gold King himself is implied to have his suspicions about Mizuchi, as he regularly sends his ‘Rabbits’ to the facility to check for suspicious activities, but he hasn’t been able to find anything incriminating yet.
Mizuchi chose Anna as a King candidate, because she is an extremely powerful Strain. He believes that her power to ‘project’ herself into various objects and ‘synchronize’ with them to ‘see’ their true nature is almost on par with the Colorless King, Miwa Ichigen. Mizuchi’s plan was for Anna to ‘see’ into the Dresden Slate and ‘synchronize’ with it in hopes of getting a reaction and forcing the Slate to select her as the new Blue King. However, all his attempts have failed spectacularly, as the memories embedded in the Dresden Slate prove too much for Anna to handle, rejecting her consciousness each and every time. In the process, Mizuchi discovered that Anna’s sensory powers are enhanced by pain, so as his desperation grew, his methods also became more extreme, and by the end of the novel he was half-drowning Anna in hopes that the pain of almost suffocating will push her abilities to newer heights. Of course, any injuries Anna sustained as a result of his brutal treatment simply got healed by his own power, so she doesn’t even have any physical scars to show by the end of this whole ordeal.
In the end HOMRA crashes the facility to rescue her, and she’s officially adopted into the clan.
