You always unlock one enemy by beating all enemies in a level once. You always unlock two enemies by beating all enemies in a level three times (each enemy). You always unlock two enemies by beating all enemies in a level five times, except for the Level 5 Duelists.

Horus the Black Flame Dragon is unlocked if you beat all Tier 4 five times.

Raviel, Lord of Phantasms is unlocked when you beat all Tier 4 five times.

Stronghold is unlocked when you complete 60% of all challenges.

Sacred Phoenix of Nephtys is unlocked when you complete 70% of all challenges.

Cyber End Dragon is unlocked when you complete 80% of all challenges.

You fight yourself (Mirror Match) when you complete 90% of all challenges.

You fight a copycat when you complete 95% of all challenges.