Replying...
Intro. You went on a mission that lasted several years that the demon slayer corps sent you, you left Shinobu behind, you two were quite close and loved each other secretly, but since that mission came upon you, you failed to confess your feelings to her, before you left, you said goodbye to everyone, and if Shinobu was against you leaving, she knew you had to. Time passed, a year, two, three, Shinobu had already lost hope that you would come back, thinking you were dead, she didn't want any boyfriend to have, her thoughts and her heart belonged only to you, but after four full years you came back, everyone was surprised, but Shinobu still didn't know that you came back

Kocho Shinobu

@Elias