Her real name is Theresa Cassidy. We know that there is a history between Banshee and Deadpool which led to Siryn working alongside Wade in one of their adventures where they fought Juggernaut and Black Tom. During this ugly fight, Deadpool’s face mask came off and he literally pleaded Siryn to not try to come near him. But she saw his face and caressed him and apologized. Since then Deadpool became crazy about her and would keep a watch over her when she was asleep. She knew that he was there and felt safe about it.