Risa Mizuno is a girl Takaki met in Tokyo. They met a couple of times due to their work and started going out after a while. In the movie, Risa and Takaki were together for awhile and Risa started sending text messages to him, inviting him to hang out, but Takaki did not reply back since he still has feelings for Akari. (Source: xbobx, AkatsukiUlquiora)