Jillian met Wanda when she was captured by Haffaton. At the time, Wanda was addicted to Heroine buds and required by Olive Branch to torture prisoners like Jillian in order to turn them. Wanda eventually helped Jillian to defeat Olive Branch, after gaining permission from Warlady Judy to change sides and join Faq. Jillian then saved Wanda's life, by making a deal with Charlie, who helped cure Wanda of most of the physical effects of her addiction to Heroine buds.
It is implied in Book 2 – Text Updates 010 that before the fall of FAQ Jillian and Wanda were romantically involved. They appear to still have had a romantic and very intimate relationship during the times when Jillian was captured by Gobwin Knob, but it's unclear how much of that is genuine, how much was the spell of control Wanda cast on Jillian, and whether there might be other factors involved. This relationship is implied to have originated from Faq during a flashback. It is unlikely Jillian knows that Wanda was unintentionally responsible for the destruction of her side.
When Jillian breaks free from Wanda's Suggestion Spell, Wanda absorbs the full damage from the spell's abrupt termination. It's possible that she absorbed this damage deliberately, in order to avoid harming Jillian. She asked Parson not to croak Jillian, immediately after the spell broke. Jillian for her part remained concerned for Wanda even after breaking free of Wanda's spell.
Jillian has repeatedly attempted to persuade Wanda to leave Stanley's side, once while being held captive at Gobwin Knob, again during a fly-by visit to Gobwin Knob before the Coalition attack, and yet again when they parleyed at Spacerock. Her attempts to influence Wanda was hindered by the fact that she didn't know certain critical facts about Wanda. Jillian did not know about Wanda's role in the First Battle of Faq or the prediction that Wanda would someday gain an Arkentool. Wanda did mention the Summon Perfect Warlord Spell in passing after she interrogated Jillian at Gobwin Knob, but Jillian still doesn't know many critical facts about Wanda's relationship with Parson.
While Wanda benefited during the Battle for Gobwin Knob from having secrets that Jillian didn't know, Jillian has secrets unknown to Wanda. Wanda probably did not know that Jillian had rebuilt Faq and become a Queen, until she decrypted Ossomer. She still doesn't know that Jillian's Royal Heir will pop fairly soon, or that Jillian has formed separate alliances with the Royal Crown Coalition, the Western Giants, and Charlie.
The two met up again at Battle of the Expository Bridge. Wanda believed that Jillian had come all this way for her, but Parson suggested that it might have been for Ansom. Reacting jealously, Wanda had Ansom sent off with the ground troops to prevent Jillian from seeing him. Wanda proceeded to parley with Jillian.
Jillian indicated that she was fond of Wanda, but opposed her side because she can't stand Stanley. Jillian offered to let Wanda join her and conquer the world together. Wanda initially rejected this offer, saying that she is loyal to Fate. The Arkentools are Fated to be brought together, and she needs to stay by Stanley's side to help this event occur. Jillian suggested that Wanda could still fulfill her Fate if she joined Jillian's side, but was under non-disclosure, preventing her from sharing details. Wanda indicated that she might take her up on this offer, if she could win back her trust by helping her defeat Spacerock. Jillian pretended to agree, but proceeded to force an end to Gobwin Knob's turn, placing Wanda in a dire situation.
Though unwilling to croak Wanda herself, Jillian took a great deal of satisfaction in leaving her to her apparent doom. Much to Wanda's shock, Jillian left Wanda to croak while stating that she was finally sure who she had come for: Ansom.
Wanda has since entered a strong state of cognitive dissonance. She is not quite able to acknowledge what has happened in her relationship with Jillian and reacts violently to Jack's pointed hints, but realizes that somewhere, she has made a mistake.