We frequently interact with others daily and experience a sense of joint agency—the feeling of performing an action together. Recent studies suggest that this sense of joint agency is influenced by the perceived "human-likeness" of partner. This study examined how a partner's behavioral process, specifically adaptation and fluctuation, affects joint agency in a cooperative task mediated by human-likeness. Participants completed a cursor-tracing task simulating collaboration, with cursor movement determined by combining their input with pre-recorded data. In this experiment, adaptation was approximated by preprogrammed changes in the cursor movement. The results revealed that adaptation enhanced joint agency, whereas fluctuation had no significant effect. Human-likeness is thus positively correlated with joint agency. Moreover, individual traits such as extraversion and attachment shaped these perceptions in unexpected ways. Poor task performance increases joint agency. These findings contribute to this field by identifying factors that influence the sense of joint agency.