The show is an ensemble piece through and through- I went to see it a few weeks agoand genuinely couldn't tell you which part was "main" as truthfully I think they're all about the same. I'm very very sleepy just now so I may count this wrong but according to my programme Steph sings 3 songs solo, 2 duets, 3 in a small group, 2 as a small group back up singer (as in, one performer sings "with" x, y, and Steph, if that makes sense!

), and 7 are Company songs. So she sings in, er, 17 out of 40 songs and I think there are 9 performers (4 men, 5 women). I haven't really got the energy right now to count up how many songs each performer is in but scanning the list it looks like only one other woman sings as many solos as Steph does so I suppose it's as much as a main part as you get in this kind of show. In all honesty though, it's very much a group effort. I went when the show was still in previews and the performers looked like they were having a great time with the show; the audience certainly did! I'm sure Jessica will love it, it's a great little show. Brilliant opportunity for her before going into training as well. I hope someone from here sees her in it and can report back! I can't really justify going to see it for a second time...
ETA: Forgot to say, with it being an ensemble show I don't think there are named parts as such. I looked the soundtrack up once I got back and the division of the songs doesn't completely tally with what's in my programme so I'm guessing the director and producer have a fair amount of freedom over the piece. Steph plays one of the young girls in a group of friends who are reminiscing about their friendships and relationships, I think. The storyline was pretty loose and it's been a few weeks since I saw it now! Certainly the programme lists the performers' names, and character names don't seem to be written anywhere.