The MSJ-121 Demi Trainer (デミトレーナー Demitorēnā?) is a mobile suit introduced in Mobile Suit Gundam the Witch from Mercury.

Demi Trainers were developed for students of the Asticassia School of Technology, a school created for training personnel in the space development industry. The unit's design is stripped of all superfluous components, affording it superb maneuverability and serviceability. The MS can be modified by anyone with basic technical knowledge, giving it excellent potential for customization. In addition to these facets, Demi Trainers are said to be the best choice for those learning to pilot, making them an obvious choice for students at the school. If asked which MS they felt most comfortable with, students are most likely to answer, "Demi Trainer".