My headcanon is that Levi has an undercut because his hair is insanely thick. Having so much hair while living in the Underground must've felt so suffocating, especially with the mixture of humidity and filth clinging onto his skin despite his best efforts. What started as a means for practicality ended up being his signature style. It was one of the few things that he felt he could still control despite the twists and turns of life.
Now, the undercut feels almost comforting. The cool breeze of spring filters through his dark locks with more ease. It feels light on summer days.
Post-war, the undercut grew out a bit during his recovery. He hardly trusted others would fix his hair with the cleanliness he did, so he opted for waiting until he had healed just enough to do it himself. Once he was finally able to, he felt a little more like himself. A little bit more human. A little bit better.
