Vex is back! He is so charming in a sexy, but creepy killer kinda way. When he isn't "controlling" others, he is even oddly likeable. It's too bad that he uses his power for evil.
If Dyson and Bo weren't so good together, I could see her being able to tame Vex. It would take some time, but Bo could do it. Maybe if Vex sticks around she can rub off on him in a positive way.
In "Faetal Justice," Dyson went to Vex's bar and got into a fight with a Dark Fae, Ba'al. After leaving the bar, Dyson woke up in the alley, bloodied... with Ba'al dead. And, with that, Dyson became Bo's client.
She would not believe that he killed Ba'al. Her trust and faith in Dyson, while admirable, is going to cause her great pain eventually. Whenever she puts that kind of trust in him, it's difficult not to foresee what will happen when she finds out that he has been keeping the truth from her. Bo still isn't over Lauren's betrayal and that was minor in comparison to Dyson's secret.
Dyson's situation provided further insight into the differences between the human world that Bo has known and the Fae world, especially in their justice systems. This drove Bo to risk herself in order to prove Dyson's innocence.
She went unarmed to Vex's club and he took full advantage of it. He saw Bo as a fun play thing and made her touch herself in a seductive way. She fought back, but that only encouraged him more. He almost killed her, but he eventually backed off. Was it because they were in public? Or does he like having her around? That was unclear, but they do have an odd chemistry with each other.
What this situation proved is that the difference between the Light and Dark Fae is not a matter of good versus evil. The Light Fae were not a positive force in this situation. The Morrigan used it to her advantage to find out more about Bo.
She didn't want justice for Ba'al or the Dark Fae, she wanted to use Dyson to find out the truth about Bo. She manipulated the situation in her favor and even resorted to torture. Her treatment of Dyson is not going to encourage Bo to join the Light Fae. Though there probably isn't anything that would push her into choosing a side. Many of the Fae she has met, including at Vex's bar, are envious of her non-allegiance.
Luckily, Bo and Kenzi figured out that Vex's bartender was the wolf that killed Ba'al, not Dyson. The Morrigan had no choice but to let Dyson go and clear his name.
Bo and Dyson's relationship is constantly changing. They get together, then pull away from each other, but since their initial meeting are continually drawn back together. While their relationship is complicated, it can only get worse as the truth comes out.