I don't know if this was known among the GTAforums-community as I'm quite new here, but I didn't see any topic or information like this so I wanted to share it.
It's possible to unlock an amazing fifteen additional playable characters in the GBC-version of GTA1 through the use of cheats. I made a video about it a while back.
As I'm a writer for a Dutch Wiki I also placed screenshots of the characters on a page on said wiki, you can find them here (the page is in Dutch).
I hope this isn't considered ''promoting my videos'', as that's far from my attention (I barely uploaded any video's on Youtube anyway). This topic was made to share some information and maybe it's new to somebody. Have a good day!
Edit: Rumors say the secret characters resemble the crew of Tarantula Studios (now R* Lincoln), the developer which made the GBC-ports of both GTA1 and 2. I guess it could be possible as Kelly is based of Kelly Sumner, who later became the CEO of Take 2 Interactive. However, I didn't manage to track any clues to verify or disprove the rumors.