The catalyst for Juri’s current broken state of mind, she simultaneously idolizes Juri for her brilliance and feels envious of her for it. Her extreme lack of self-worth is the reason for the rocky relationship between the two.
Shiori was ironically also in love with Juri the entire time but she believes Juri’s affection towards her was out of pity, not love. She thus wants to remain in control of their relationship by never confessing to Juri so she will be the winner of their rivalry. In the movie, she is the main antagonist who manipulates Juri’s feelings for her so Juri can duel with Utena.