What do you call the lead developer's offsider?

I just realised that my resume needs to be updated, because the project I've been working on has hit a major milestone. An amateur computer programmer done good, I'm a subcontractor for one of the big guys. Basically I'm the principal contractor's (who is lead developer, primary project manager and domain expert) offsider. How should I describe that in less than 5 words in my resume?