Leen first meets Touya after Paula leads him away from everyone else during his first time in Mismede. Leen initially wanted Touya to be her apprentice also, but he declined. Leen generally gets her way with Touya, whether it's making him read minds with Recall, making him open a gate to Eashen, or simply just teasing him. In Volume 8, she confessed her feelings to Touya and offered to be his 8th bride.
Charlotte is her apprentice, Leen gets told everything, sometimes even by force (above quote). Charlotte is fortunate to be her student, they both say since Charlotte can not defy her teacher. Charlotte is afraid to upset Leen and usually says she's 'joking' if she does it by mistake.