training partner
Used in traditional arts (e.g., dance, music, martial arts) for a person who practices with you, often as a peer or junior.
In Japanese dance practice, she acted as my training partner.
training hall; practice platform
Especially a wooden-floored hall or raised platform used for dance or traditional performing arts practice.
This training hall has a wooden floor and is used for dance practice.
相方 is a partner in a performance or act, often in comedy or music, while 稽古台 is specifically a training partner in practice.
Compound of 稽古 (keiko, 'practice') and 台 (dai, 'platform/stand'). The 'training partner' sense likely derives from the idea of a person serving as a 'platform' for practice.