Okay, so an argument could be made that Rab Silsbee is the next most important character, but our vote is for Cilla. Here's why.
Being there—being present when needed—is a huge part of Cilla's character. When John Hancock rolls up, and all the Laphams are freaking out, it's "Cilla who thought to offer him her apron for a towel" (1.3.14). She's there when Johnny needs to tell her about the Lyte cup, when he burns his hand, when he's acting like he doesn't need anybody, when he decides to burn the Lyte genealogy and leave the cup behind. She's not there only for Johnny, though. Her devotion to her little sister, Isannah, is consistent, even when Isannah treats her like dirt. Cilla as a whole is consistent. She's dependable—if Cilla Lapham says she's going to do something, she's going to do it. Look at her insistence on closing up the Lytes's house properly. She doesn't even like the Lytes, but it's her job, and she's going to get it done. Hmm. She's a little like Johnny in that regard.
Cilla may be dependable, but she's no pushover. In fact, she's constantly giving Johnny the reality checks he needs, whether it's refusing to be overawed by his talent, confirming through her actions that people still love him after his accident, or calmly noting his standing her up one too many times and letting Rab walk her home and take her out. "Cilla had changed so much Johnny felt confused. One thing was certain. She wasn't going to hang around and wait for him either on street corners or at back doors—and then not have him show up" (7.2.51). You go, Cilla. You go.
That's an important quality in a book set in the Revolutionary era, wouldn't you say? Whether it's holding on to people or nations, Cilla understands that you've got to know when to hold 'em and know when to fold 'em. We see this early on when she's skeptical about the value of Johnny's relationship to the Lyte family. It comes out later when she makes efforts to stay in touch with Johnny even after he leaves the Laphams's household and their engagement is broken, but also refuses to put more into their relationship than he does. Ultimately, we see this quality when Cilla loses her beloved Isannah to Lavinia Lyte, realizing that Isannah is lost to her and that crying about it isn't going to do any good. "Cilla was not taking the parting as hard as Johnny would have expected… She had lost Isannah long ago" (11.4.33).