Sinclair is the secondary antagonist in Art of Fighting 3.
Sinclair is working with Wyler, pretending to encourage him in his experiments, but secretly asking him to stop them; it is unclear whether she will assassinate him there. She eventually faced Wyler and apparently wanted to test the efficiency of the drugs he took for himself.
Sinclair did not disclose her personal reasons and answered that Wyler "telling you won't make a difference". After his defeated, Sinclair left him and did whatever he wanted.
It is worth noting that Wyler did not return to his childlike mental state as he did in other endings; the effect of immortality is still active.
Sinclair is a mystery to all -even Wyler- as she reveals little about herself.