Hatsun appears in the warehouse district shortly after Silver completes his ninth Town Mission, "Protect the Barrels in the Warehouse District!", in which he must defend explosive barrels for a warehouse worker named Roberta in New City, Soleanna. Hatsun only appears at this very portion of the game, will be gone after the player progresses further with anything, and in no other storyline. He plays no role in the story whatsoever and his only line is "coo".
Hatsun is the only non-anthropomorphic animal in the game who can be spoken to.
If the player does the wall glitch with Shadow in the SoleannaCastle Town after completing Mission 15, the player can enter into a different version of Soleanna New City where the player can also see Hatsun in the Warehouse District as Shadow rather than Silver.
Sonic the Hedgehog (2006) XBLA demo contains many leftovers, including message texts with four other Hatsun's lines:
"What's your problem? I'm a normal pigeon. Wanna make something of it? I'm a totally normal pigeon. Check this out -- Coo, coo... What? What do you mean pigeons can't talk!? Uh oh... Coo..."
"Coo... Hey, you'd better watch it. Soleanna's pretty laid back place, but you still don't see many people talking to pigeons. If you don't want to get yourself locked up somewhere with soft walls, just turn around and go home! It was nice to have someone talk to me for a change, though. Thanks... Coo..."
"Coo... Hey, you! Just leave me alone! The other pigeons will get suspicious! Go away! Don't make me angry, or else... Uh, oh... <Pigeon mech "Hatsun" restarted in pigeon emulation mode>... Coo..."
"Coo... Hey, you, hedgehog! Yeah, you! You've got some nerve, talking to me! You'd better learn to keep to yourself a little more, you know? You're just asking for trouble by hanging around here. Listen, this is for your own good. I'm not saying this because I don't like you. I like you, okay? I like you a lot. Coo..."