Here is an updated list of who are Keyblade Masters and who are their apprentices.
Master Aqua
User: Rain Fall Master
Trained by Master Eraqus
Past Apprentices: Riku, Ventus, Sora, Mei
Current Apprentices: Faye
Master Riku
User: Power of Darkness
Trained by Master Aqua
Past Apprentices: Hina
Current Apprentices: na
Master Ventus
User: Wind Apprentice
Trained by Master Aqua
Past Apprentices: na
Current Apprentices: Valsing
Master Roxas
User: Roxas 69
Trained by Master Hina
Past Apprentices: na
Current Apprentices: na
Master Sora
User: Wielder of Light
Trained by Master Aqua
Past Apprentices: na
Current Apprentices: Rina
Master Mei
User: LoveKouichi
Trained by Master Aqua
Past Apprentices: Chero
Current Apprentices: na
Master Hina
User: Tiggerola
Trained by Master Riku
Past Apprentices: Roxas
Current Apprentices: Aetas
Master Chero
User: MewChero
Trained by Master Mei
Past Apprentices: na
Current Apprentices: Kagami Uchi