There are 14 secret characters in Mario Kart Wii which can be unlocked. Below is a list of characters with how to unlock them.

1. Birdo: You need to play 16 different courses in the Time Trials mode
2. Baby Daisy: Play all the 50cc cups and get at least a “One Star” rank in all of them
3. Bowser Jr.: Play all the 100cc Retro Cups and get at least a “One Star” rank in all of them
4. Diddy Kong: Successfully complete all 50cc cups
5. Dry Bowser: Play all the 150cc cups and get at least a “One Star” rank in all of them
6. Daisy: Get first place in the 150cc Special Cup
7. Dry Bones: Get first place in the 100cc Leaf Cup
8. Baby Luigi: First unlock the Expert Staff Ghost data by beating a regular staff by 7 seconds. Then unlock eight expert staff ghost data in Time Trials.
9. Funky Kong: Unlock 4 Expert Staff Ghost Data in Time Trials Mode.
10. Mii (Outfit A): Get first place in the 100cc Special Cup
11. Mii (Outfit B): You will have to unlock all of the 32 Expert Staff Ghost Data in the Time Trial mode. To unlock Expert Staff Ghost data, you will have to beat the regular staff by 7 seconds.
12. King Boo: Get first place in the 50cc Star Cup
13. Toadette: Just play all 32 courses in the Time Trial Mode. It can also be unlocked by winning 1,000 Wi-Fi Online matches.
14. Rosalina: Play all the Mirror Mode Wii-Cups and get at least a “One Star” in all of them.

More stuff that you can unlock on Mario Kart Wii
Bikes
- Quacker (small bike) – Get first place in the 150cc Star Cup
- Magikruiser (small bike) – Play Time Trials on 8 different courses
- Dolphin Dasher (medium bike) – Get first place in the Mirror Star Cup
- Rapide (medium bike) – Get first place in the 100cc Lightning Cup
- Nitrocycle (medium bike) – Play all 100cc Wii Grand Prix Cupsand get atleat a “One Star” rank in all of them
- Bubble Bike (small bike) – Get first place in the Mirror Leaf Cup
- Phantom (large bike) – Get first place in the Mirror Special Cup
- Twinkle Star (large bike) – Get first place in the 100cc Star Cup
Karts
- Cheep Charger (small kart) – Play all 50cc Retro Grand Prix Cupsand get at least a “One Star” rank in all of them
- Rally Romper (small kart) – Unlock 1 Expert Staff Ghost Data in Time Trials
Blue Falcon (small kart) – Get first place in Mirror Lightning Cup
- Turbo Blooper (medium kart) – Get first place in 50cc Leaf Cup
B-Dasher (medium kart) – Unlock 24 Expert Staff Ghost data
- Royal Racer (medium kart) – Get first place in 150cc Leaf Cup
- Piranha Prowler (large kart) – Get first place in 50cc Special Cup
Aero Glider (large kart) – Play all 150cc Retro Grand Prix Cups and get at least “One Star” in all of them
- Dragonetti (large kart) – Get first place in 150cc Lightning Cup