MAJOR SPOILERS!!!
MAJOR SPOILERS!!!
You did do something wrong. There is an official walkthrough among the game downloads, it should help you out. Here are the criteria for the best ending:
- You must free Mike from prison
- You must not accuse Mike at the Yakuza meeting
- Either don't buy the Angel's Breath potion or, if you did, do not take Astaroth's hand during the trip
- Tell Asmodeus you'll leave noone behind
- Shoot Asmodeus
- Ask for Mike's help for the finale
- Do not leave the sewers during the finale
- Refuse Astaroth's offer during the finale
- Ideally you are on every girl's path at the end, but you can get the best ending with any configuration of girls (EDIT: you need at least 2)