I'd just just use the noble statblock, bumping up the hitdice if I thought he needed more.
And then as the game progresses, when the players wind up teaching him something, I'd add that to his statblock ( and maybe adding more hit points, but probably not).
For example: If the PC rogue is regularly getting his sneak attack because of the noble's positioning or help, then I'd add Sneak Attack to the noble's statblock. Or if he's always fighting beside the PC battlemaster, I'd give the noble a Superiority Die and one of the maneuvers the PC uses a lot.
Just imagine how fun this would be if your players are barbarians and warlocks!