Charlie and Jillian are both mercenaries, and they both tend to honor the letter of their contracts, while breaking the spirit of said contracts when it is in their interest to do so. One of Charlie's Rules reminds his Archons that by making trouble for their clients, they also make more business for Charlie. Jillian worked for the First Royal Crown Coalition, while secretly meeting with Wanda.
While we don't know the exact wording of the deal Charlie made with Jillian, it seems that he agreed to aid Jillian in return for her help in dealing with the threat posed by Wanda. Charlie expected Jillian to croak Wanda, but Jillian tried instead to persuade Wanda to break with Stanley and join her. While having Jillian and Wanda join forces would not have been what Charlie or Don had in mind when they decided to help Jillian, Jillian could argue that recruiting Wanda would have made Wanda less of a threat to Charlie, and would have done a great deal to weaken Gobwin Knob, which were the main goals they had in mind when they decided to help her.
Since Wanda refused to change sides, Jillian had Vanna end Gobwin Knob's turn. Since Jillian believed that this act by itself insured that Wanda's force would be defeated, she regarded it as being sufficient in itself to fulfill the terms of her agreement with Charlie to end the threat posed by Wanda. At this point, she decided to perform acts that would strengthen Faq at Gobwin Knob's expense, such as capturing Ansom and sacking some of Gobwin Knob's cities, while avoiding those acts that would weaken Faq, such as launching a frontal attack against Wanda's superior forces.
Charlie, however, did not think that Jillian had ended the threat posed by Wanda, because he, unlike Jillian, knows that Parson has the ability to find a way that Wanda can escape the trap Jillian placed her in. Charlie's anger at Jillian's failure to personally croak Wanda lead him to try to have Ansom croaked by Sammy before Jillian had a chance to capture him.
Jillian may have fought against Archons on occasion, before the Battle of Gobwin Knob, and may have sometimes fought alongside them, if her clients ever hired both her and Charlie. Given their sides' extremely different approaches to mercenary work (Charlie is very well known and works hard to further his reputation; Faq had a history of being veiled and took steps to have no local reputation at all), it is unclear how often they competed for the same contracts or were hired by opposing clients. However, Jillian worked far afield when Faq was still standing (to protect the side's anonymity), and Charlie deals with sides both near and distant, so the two of them are probably the most "worldly" participants in the conflict with Gobwin Knob, at least among non-Casters.
Jillian never trusted Charlie, but did talk to him often enough to make taking his calls habitual.
As a former mercenary, Jillian probably understands Charlie's mercenary mindset better than any other Royal, which may be why she doesn't assume that Charlie and Stanley would become allies based on a common anti-Royal ideology. As a former Barbarian, she is also much more willing to take calculated risks than most other Royals, and so is willing to risk dealing with Charlie. She's confident enough to believe she has a good chance of benefiting from the association.
As long as Stanley and all of the other members of the Royal Crown Coalition II refuse to deal with him, Charlie needs Jillian badly. He's likely to be careful not to antagonize her, because he probably knows how easily provoked she was as a Barbarian. He knows she leads the newest and probably weakest member of the RCC II. If he wants her to be able to influence the decisions of the RCC II, he needs to help her build up the power and influence of Faq as quickly as possible. Fortunately, Jillian is willing to try Charlie's unconventional ideas, such as persuading the Western Giants to switch their loyalties from Jitterati to Faq. Charlie probably knows that Faq obtained much of its income from doing mercenary work, and may be concerned that it could eventually become a competitor once again, but given the magnitude of the crisis he faces, that possibility is likely to be of comparatively small importance.
Jillian's sudden decision to change their agreed upon plan at Jetstone and not attack Wanda placed some stress on Jillian's alliance with Charlie, but Jillian's revelation that Wanda does not hate Charlie and would probably be willing to work for him forced Charlie to reassess his previously held desire to have Wanda croaked. Charlie's alliance with Jillian may actually be strengthened if Charlie concludes that Jillian was right not to croak Wanda under these newly revealed circumstances.