Have you checked if you unlocked the "No More Strangers"
achievement / trophy? That'll tell if you really left a random character undone or maybe there's something else. Here's something I found related to when he'll appear.Eddie Low - First EcounterTime: 20:00 - 4:00
Task: Take him to the docks. Drive him home.Eddie Low - Second EcounterTime: 20:00 - 4:00
Task: Kill Eddie (he will attack you with a knife)
Try also looking at the 100% Checklist
If you still can't find out what you haven't done, try uploading your save file here
. If you use xliveless, you should find your save file at C:\Documents and Settings\<username>\Local Settings\Application Data\Rockstar Games\GTA IV\savegames