Why in god's name am I only seeing this now. Christ alive o(-(

Anyway, in case you're somehow still wondering a literal year later--Dieter had long hair like that before he came to Eorzea, but cut it in Ul'dah as a concession to the climate. He makes the initial decision to grow it out again after EW as part of his campaign to emotionally recuperate from the events. He couldn't get his hair any shorter, so he went long instead, & he skipped the awkward in-between phase of hair growth through the wonders of alchemy. It did not make him feel meaningfully better, but he's continued to grow it out anyway

(Thank you for asking & for real, I am so sorry lmao)