Just Speculation...
It's been ages since I've participated on here and I feel I should jump back in before I lose my Pros license. :-P
As you guys know, Doyle is my favourite character, but in the light of certain personal events, I've started to wonder a bit more about him. He's energetic and reckless, mercurial in emotions and has incredibly devastating guilt complexes. He's artistic and seems to have less sense of danger than Bodie! All of these *could* be signs of him having undiagnosed ADHD. People with ADHD experience rejection and failure on a 'higher' intensity than neurotypicals and - if checked as adults - can often be misdiagnosed with bipolar disorder because of the emotional similarities. This is more common in females, however!
The more 'obvious' traits are the energy and the recklessness. I can't remember if his flats are messy, but that's often a trait as well.
Probably barking up the wrong tree, but I just thought I'd put it out there. :-)