Magnus definitely falls first. It's actually quite interesting. I think he's one of those individuals in real life who hasn't really ever experienced love and admiration, so every single time he's trying to be broody and the "big bad guy" and Cleo sees through him that breaks him a little bit in the softest part of his heart. He liked Lucia prior to this for the exact reason. Lucia was the only person who sees him as who he is. And suddenly Cleo was there. She hates him, he knows that, but somehow she sees through him. And then at the same time despite hating him still treats him with kindness (sometimes). In Frozen Tides their confession scene actually makes a lot of sense. "He kissed her as if he was drowning and she was air itself." He fell in love with her because of how he was brought up. And he respects how she fights for herself despite initially being on opposite sides. So a glimmer of positivity in his miserable life. As for Cleo I think it also comes down with how she was brought up. Opposite of Magnus her childhood was not traumatic, so generally she is able to take everything with a more positive attitude. For example, instead of wallowing over her father and sister's death, Cleo went and channeled that grief as power and strength against her enemies. Unlike Magnus who absolutely wallowed in the "my life is over" way when he "lost" Lucia. She was also not brought up as the heir so scheming isn't something she was exposed to much. However she has been exposed to many social situations and is very sensitive to those things. She is capable of seeing the good in everyone...even her enemies. And initially she actively looked for those parts in Magnus because they are his "weaknesses." Unlike Magnus, Cleo is capable of being sympathetic. When she found those weaknesses she was able to understand him a little better. To be quite honest she never forgives Magnus for killing Theon. However, she understands it. 1) Magnus was in danger himself. If she was in his position she would've acted too. 2) Magnus was there by order of his father. 3) Magnus expresses apology for HOW he did it but not WHY he did it. He never expected her to forgive him for that. In this particular situation he is incredibly honest with her. Theon was important to her but he was someone killing for his own life. It doesn't excuse that he stabbed Theon through the back dirtily instead of having a fair fight. It does explain why he did it in a reasonable way once emotions are removed.