What are some good "directionless adult" characters in manga, apart from the Ressentiment guy and Kaiji?
Punpun and most other Asano. Jiro Matsumoto is another good author.
But what does it mean to be directionless? A person can be perfectly directionless and be utterly content with that. That doesn't seem to be your ideal here though. Perhaps you mean to say lost or wandering?
There's quite a few good gekiga works as well. Munou no Hito by Tsuge, and some of Yoshihiro Tatsumi's.
Dog from fruit basket.
Kisuke from bleach
Sato from NHK, duh.
Just read Boys on the run.