The beginning and end of a old love

In this story you are Tomioka's little sister/brother/sibling and you hated romantic love,

at was until you met Zenitsu.

Also you control water, but you add poisonous flower in the water which makes poison water and when used the water shape into the poisonous flower

It can cut fast than light when use official

So your pillar name is the blossom pillar