Banner carriers in battle are for your troops only, one troop will carry your banner.
There is no item in-game like a banner.It's simply a nice addition which gives atmosphere.
To get out of your faction simply talk to your king and tell him
"I'm tired of being your vassal i want to retire", something like that
- i do'nt quite remember the dialogue. But it isn't difficult.
If you are a mercenary you have to wait until the contract is finished.
Then choose "don't renown the contract with faction "X""
and you are free to go.
PS: Probably banner-carriers are for lords only not mercenaries, though not sure