Onishima is shown to be exceptionally short tempered and slightly maniacal, sending whole armed platoons after the GG's and other gangs, ranging from Officers, to police dogs, to helicopters armed with missiles, to SWAT, to parachutists with machine guns. He himself uses a large revolver to take out gang members. When the Golden Rhino's show up, DJ Professor K specifically says in a cutscene that the police force was "too scared to take on the Golden Rhino's". This may be why Onishima does not appear again after chapter 1.
Onishima is a middle-aged man, wearing a beige detective coat over a white shirt with a crumpled red tie. His pants are brown and he typically carries an oversized revolver, which parodies -as does the whole character- Clint Eastwood's character Harry 'Dirty Harry' Callahan, from the eponymous film series. Like the rest of him, Onishima's hair looks unkempt and greasy. Ironically, his hairstyle is usually associated with delinquents in Japan.
- In Sega Superstar Tennis, when Beat wins a game, he'll sometimes say: "Captain Onishima failed to stop me, just like you".
- He is also mentioned on Beat's bio in Sonic & Sega All Stars Racing.
- Captain Onishima does not have an walking animation, as can clearly be seen in the Benten Boogie level.
- Captain Onishima possess similar characteristics and a similar role to Inspector Zenigata from Lupin the Third