He said "We are SO not going there." to Sophie so I think it can't be Cassius because he acts as if it is embarrassing, and he would be ashamed of Cassius not embarrassed.
I think it's something to do with Sophie like maybe i dont know, Keefe Foster Sencen? That would be embarrasing because his middle name is related to Sophie and well, its just embarrasing i guess
I bet we are never going to know. The author will probably keep it secret, if she even has one in mind, and it will drive us crazy til the end of time.
If you look on keefe's registry file, it says "keefe [Im not your legacy boy] sencen" so I honestly wouldn't be surprised if legacy is his middle name.