Mark was Emma and Melissa's (Curtis' female form) running coach. He drugged them both and tried to rape them. He brought Melissa into his car, and began to undress himself. Within this time period, Melissa changesd back into Curtis and punched Mark when he realised what he was trying to do.
He also tried the same procedure with Emma, but Curtis realised what was happening and ran to look for him. He hit him once again and met Emma when she woke up and told her what happened. Emma and Curtis tied him up naked on the running track, writing "I DRUG AND RAPE GIRLS" on his chest.