One of my players is planning on using one of these in a game I'm going to start next week, but from what I'm seeing this subclass is kind of lackluster. It takes on a supportive combat role and enables a social face role, but mechanically it looks to be more boring than even a champion.

They'll be starting at level 3. I know two of my other players want to use a battle master and a cavalier. The other two are going life cleric and glamour bard. How can I make a banneret fit in without being outshined by the others?

Edit: The bard is taking the criminal background, so they aren't completely hosed by locks and traps. PDK is a noble, BM is a soldier, cavalier is a variant noble (knight), and cleric is an acolyte.

Is it a new player, or an experienced one? 
If the player has experience, roll with it. They know what they’re getting into. They looked at PDK and said to themself, that looks fun, no need to second-guess them. 
If they player is new, maybe point out the parts of the class you think are weaker, just so they understand. If, for example, they really want to play a knight, and they went with PDK because it has knight in the name, explain there’s other ways they can flavor their character, and being a knight can be a backstory choice; it doesn’t have to specifically be in the name. If you tell them and they still want to go with it, let them. As long as they have fun, that’s all that matters. 

And if they do go with it, and you’re still worried, make sure to give them a chance to shine. Make their knighthood important sometimes, make commoners, for example, so very impressed that a knight, and a noble to boot, is willing to help them. Make their character special in the way the world treats them, even if they won’t be as flashy in combat. 

If you're willing to homebrew a little bit, I'd suggest moving Inspiring Surge to level 3 and Rallying Cry to level 10.

1. It’s a support fighter.  Second Wind/Rallying Cry can heal for 1d10+level, and an additional level*3.  That’s an average of 17.5 at level 4, plus 4 for every level thereafter.  Compare it to Lay on Hands:  lower amount of healing, but it’s ranged instead of touch, bonus action instead of action, and replenishes on short rest not long rest.
2. It’s a face at level 7 if you want. Unfortunately you also have a bard in the group so that will overshadow the PDK a bit.  Otherwise, the PDK would be a very respectable backup negotiator.
3. Inspiring Surge is much more valuable than you think - but only if you have a heavy hitter like a barbarian or rogue in the party.  And unless you get to 17/18, it’s not going to scale well without them.