He's almost never seen without Otaki. There's not much to discuss about his character, He's loud, mostly shallow and is there mostly for comedic effect - usually making Keiichi do tasks or jobs.
He, along with Otaki run the NIT Motorclub, and after the club plans for a Hill-climb race in which they were absent for (But show up at the actual race), they insititute Keiichi as the "head: of the Club, but still control it from an "Executive High Level Branch".
Tamiya used to be rivals with Otaki for Chihiro's love, but Chihiro refused both of them for the good of the club.
His most prominent hobbies, shared with Otaki, is torturing Keiichi and working on Motorcycles and Cars.
For a short while he was also in love with Peorth, during her first visit.