Background... obsession with Sou-kun, fixation, resentment...
They were childhood sweethearts who promised a future together, but broke up due to fierce opposition from his mother.
A lingering affection that hasn't faded yet, intertwined with feelings of resentment born from the sense of "betrayal" toward Sou-kun, who ultimately became obedient to his mother.
She'd probably end up playing the role of the "former fiancée who casts a dimly ominous gaze swirling with love and hate."