Stephanie first met Star when the girl got shot by a gangbanger. She took out the gang members who attacked Star by blowing out their tires, and then helped Robin dress her wound. She had to pull him away when the ambulance came.
Realizing he clearly knew Star, Stephanie believed Tim was being dishonest and possibly cheating on her in his "other life". This was the straw that broke the camel's back on the subject of not knowing who Tim really was, and she ran away and refused to speak to him.
Stephanie decided she would find out Tim's secret identity by being around Star, and pretended to be a volunteer at the hospital where Star was staying to recover, under the assumed name "Robin". She mistakenly believed Star's brother to be Tim, but was proven wrong when she followed him.
Embarrassed by her mistake, she was there when Star exited the hospital, and was decidedly more friendly to the girl in her inner musings. Star realized Steph's name was not really Robin when Stephanie's mother caught her in the hospital, however.