He's pushing his glasses up the bridge of his nose. All that sweat makes ones face slippery.

Not a backstory quirk, but I have a fun alternative mechanic for a Druid!

I have a Druid PC in the campaign I'm running right now who's still relatively new to DnD and was disappointed that Wild Shape is limited to just twice a day (the DnD movie might be partially responsible for this). After some discussion we came up with some homebrew rules that she's happy with and I've really enjoyed too.

She chooses two animals per day that she can Wild Shape into and she can switch between them and her human form as often as she'd like, but their HP doesn't reset and if one of them hits 0, she loses that Wild Shape for the day. She doesn't have to choose them at the beginning of the day, but once she's transformed into an animal, she's locked into that animal for the rest of the day. So if she decides to transform into a cat to be sneaky at the beginning of the day, she loses out on a possibly big beefy meat shield later on. She also can't choose the same animal twice.

It's been a fun way of giving her the flexibility to go in and out of animal form during RP (and sometimes a little in battle) without feeling like the balance is thrown way off.