Sugar is all there is to Ruruka Ando.
This is not a metaphor, although it works like one. It is true: Ruruka is sugary sweet, outfits pink and saccharine. Sometimes she behaves that way, too—if you're on her good side. Friendship with her is a whole lot of sweet, and little substance. But again: this is not a metaphor.
Sugar is all there is to Ruruka Ando, because talent is all there is to Ruruka Ando.
Her talent is what makes her matter. It's what connects her to people—what got her friends. It's not an unusual view point. Hope's Peak Academy pervades every aspect of everyone's life. Everyone values talent. It is all that matters, and this is true for Ruruka Ando too.
She likes being relied on for her talent. It is an affirmation of her existence. It is a reassurance, that the person will stay. To eat Ruruka's sweets is to say she matters; it is to say you value her. She views others talents the same way; to rely on them is to respect them, to ask them favors is how you say I value you without words. Relying on her talent affirms her; shouldn't it be the same for everyone else?
Her whole identity is built on sweets. It is intertwined with her.
Seiko Kimura rejects her.
She can't eat sweets. She won't eat sweets. Confectioners, by their very definition, work with sugar. There is no compromise, nothing to do. She can't tell Ruruka she values her. She can't reassure her, and so Ruruka is always afraid.
She doesn't understand her, either. She feels used when all Ruruka is trying to say is she's valued. And maybe deep down, they always knew this would explode. They are fundamentally incompatible.
In a way, Ruruka always knew Seiko would betray her.
After all, all there is to Ruruka is sugar. Who would stay for anything else?