New hot take:
Kazuki let his goatee grow bc he saw how Miri fangirled over Rei growing out his undercut.
One day Rei just decided to grow his hair out as a sign of him letting his guard down and relaxing and Kazuki was jealous how Miri kept commenting how cool Papa Rei looked now and how he looked like some famous painter whenever he had his hair up now.
Kazuki was determined to win the Super Cool Dad award.
Because he had enough hair on his head he didn't want to cut (he tried to download a bunch of apps which let him swap out his real hair with filters, didnt work, looked freaky.
Anyway, Kazuki decides that, since he didn't want to touch the hair on his head, he would use his other secret weapon.
Hsi facial hair.
And thus, the goatee was born.
Kazuki lives every single day in denial since the Goatee Reveal and he vehemently denies Miri crying about how lame Papa Kazuki looks now and Rei shaking his head cringing very badly.
Kazuki also pointedly ignored the appointment Rei, Rei of all people, made him at the hair salon with an explicit request to shave off his goatee.
Kazuki was determined that he was cooler now with his goatee.