The New Life

Toshiro and Byakuya died during the Thousand Blood War.
Toshiro died because of him being turned into a Zombie and being killed by Mayuri, whereas Byakuya died because he was killed by Toshiro.
Toshiro and Byakuya lived on in their new life.
Toshiro was a normal high school student going into a very popular and high level Shin'o High School within Karakura by her grades whereas Byakuya got into the same high school because his father got him into it by his money.
Despite different reasons of getting into the school, Toshiro and Byakuya are good friends, best friends.
Toshiro's friends kept on teasing Toshiro because they new she had a massive crush on Byakuya.
Byakuya's friends kept on asking Byakuya when he'll go out with Toshiro.
Now the Shinigami are asked to go to Karakura town to get rid of the Hollows, they never expected to meet the two former captains.
Hell they never expected one of them to be a girl and dating each other.