When I'm planning a characters story arcs I have to always leave room for adjustment due to other people planning things without me being aware of the consequences that may be brought to light, for Karina, it's actually pretty handy because she, like many others, is a product of the world around her.
It's not brought up much if at all but growing up in Hades wasn't as bad for Karina as it was for others around her. Whilst her dad wasn't around much if at all, she spent the majority of the time with her grandmother and older brother. Sure enough her childhood had bumps but nothing that could be considered "tragic".
It was only when she came face to face with the reality of the world (by finding herself within a position of power in Hades) that things really started to spiral out of control when referring to her mental stability. Above all else she was alone. Her brother being the only one she could probably trust had left her, her grandmother had left her, her father had left her. Being left to fend for herself was what ultimately kept her alive.
Karina began to see things she hadn't been aware of before, her naivety shattered like glass as she slowly realised that the world was not as black and white as she thought it to be, being in any semblance of power would automatically thrust you in a deep shade of grey that frankly she was not prepared for. Not everyone could be catered for, not everyone deserved the best and not everyone could be saved. Seeing Hades, albeit a small territory, for what it truly was had temporarily changed Karina's outlook on the world around her and drained her. Every call she made, every murder she green-lit was all for the survival of herself and this mentality of having to cope alone to survive had been etched in her brain and would only change when she finally met Eos.
When she finally met Eos, an ideal of hope was injected into her psyche, what she saw was the chance at being a "white light"; helping people that she thought deserved to be help. Whilst she harboured animosity at first, being able to help people had really changed things for her and her new found friends coming to her rescue really solidified it for her.
What she saw was a chance to right the wrongs she had done in the past and she would do anything to keep the people around her from leaving. Their goals became her goals, she would train so she could fight along side them as opposed to cowering behind them. To her Eos was an ideal of hope whilst to the rest of the world it was something to fear.
And then it all changed, her friends were taken away from her and once again she was alone.
Eos Temporary Breakup
Karina perhaps took Eos' breakup the hardest, whilst a reunion had been planned she perhaps took the breakup the hardest, once again her family had left her and one again she was left to fend for herself. She wouldn't let that get in her way though, she maintained the desire to help people and continued her training. She gained the Linus magic crest, studied numerous forms of magic, even enhanced her skills as a ninja and continued to deliver her own brand of justice.
But even after all this, she was empty inside. She'd become cold, ruthless and unapologetic like the concept of death itself, she'd do anything to feel something even if it meant sleeping with Hideki despite there being no emotional connection at all. The sense of abandonment left hanging over her head had nearly driven her to suicide and had Eos not reunited when they did, it might have succeeded.
Karina's mindset throughout her life had sort of blended together when Eos reunited. She still shared the groups goals and had grown warmer during the reunion but she still maintained this merciless attitude toward human lives and killing in general. She felt the need to take the life from anyone who she deemed a threat to Eos and this had driven quite a few edges between her and other members. Despite the fact she tried to hide it, this sense of taking someone's life whether it be necessary or not continued to be a reoccurring theme leading up to the war.
It was not until she was reunited with her brother and belittled by him, that Karina had become aware of what she had become. After that fight she had stopped killing altogether and became an advocate of peace in a similar fashion to Hitoshi, thus completing her character arc.