What did Aizen want from Rukia?

In Bleach manga and anime series. Setting up the events from the shadows, Aizen arranged for Rukia Kuchiki to meet with Ichigo Kurosaki so he can become a Soul Reaper.

Why is Rukia so important to Aizen?

Rukia thought she was an unremarkable Soul Reaper. But she was the key to Aizen’s plan, gave Ichigo his first step into the spiritual world and transformed Byakuya’s and Renji’s way of thinking for good.