At this stage their relationship still is like father and daughter but you can see the sparkles of Eddie's feeling in his words and actions. His actions seem overprotective but actually he is trying really hard not to lose Catherine. Catherine does not realize how Eddie feels and even though she realizes she does not accept it until the end of Act 1. On page 15 Eddie says to Catherine: "That you would ever grow up". That's where the sparkles of Eddie's true feelings really start to show and that's the place where Eddie starts to realize it himself but he does not know surely how he feels. He thinks his feelings are like a father's feelings. .
When Marco and Rodolpho arrive on page 16, Catherine is shocked by Rodolpho at that point you can actually feel that something is going to come up between those two. Eddie senses that something is going to happen between Catherine and Rodolpho and that's why when they chat together he usually addresses Marco only and in some cases ignores Rodolpho or cuts through his sentences. Catherine fancies Rodolpho that's why she especially tries to ask questions directly to Rodolpho. On page 21 while Rodolpho is singing Catherine gets so amazed that Eddie does not like it. Eddie attacks Catherine for no reason. He asks her to take off her high heel shoes. On those two pages 21 and 22 you can see that Eddie does not like Rodolpho, mainly because he is starting to flirt with Catherine. On page 22 at the dinner table Eddie makes a comment about the incident that just happened which was Catherine wearing high heels and Rodolpho makes a silly comment. After Rodolpho's comment in the stage direction for Eddie we can see his dislike for Rodolpho is starting to turn to hatred. The stage direction is: "he is sizing up Rodolpho, and there is a concealed suspicion". This is important in Eddie's and Catherine's relationship because Rodolpho's arrival and their relationship between Catherine is the thing that puts Eddie's feeling on the surface.