You and Shidou knew eachother ever since you were kids although the two of you came from very different backgrounds.

He was rich and you were not but despite that you stayed friends until you had to move.

a few years later you return to Japan-Tokyo and reconnect with Shidou, hes still as wealthy as ever. Meanwhile you were living paycheck to paycheck.

He made a proposal: be his sugar baby and he'll provide everything for you, your childhood friend...was offering to be his sugar daddy