A scarlet rose banner costs 18K, so I guess you're starting at a fairly high level? Unless that's a long-term ambition.
Anyway, ideally you want a race which adds to Str and Cha like the suli or angel-blooded aasimar, or human for the bonus feat/racial favored class bonus. Others are certainly possible but those put their hands up.
Power attack is your feat tax to have decent offence as a non-spellcaster. If a scarlet rose banner is far in the future you may want the flagbearer feat.
Suppose you decide on the angel-blooded aasimar and want to aim for the aasimar wing/flight feats. Feats might go
3: Combat reflexes, bodyguard B
5: angelic blood (prereq for angelic wings)
6: shield focus (prereq for shield brace) B
7: shield brace (use a shield and spear/polearm at the same time)
9: angelic flesh (prereq for metallic wings), outflank B
11: angel wings
12: missile shield B
13: metallic wings
Alternately, say you want to play a human and really focus on bodyguarding. They might go
3: shield focus, bodyguard B
5: shield brace
6: vanguard style B
7: vanguard ward
9: defended movement (counts as dodge/mobility for prereqs), outflank B
11: combat patrol
12: vanguard hustle B
13: in harm's way
Or maybe you want to play a suli who has better than average offence. That might go
3: combat reflexes, bodyguard B
5: weapon focus (longspear)
6: weapon trick (polearms) B
7: dazzling display
9: shatter defences, outflank B
11: lunge
12: disheartening display B
13: violent display
There's other possibilities, that's just a few examples.
@AVR - Not sure about PFS, but the rulebooks say that Level 8 starting gold is 33,000gp. So it'll be more than half (leaving OP 15,000gp left for Magic Armor, Weapons, and else gear). But just read the OPs newest post, and looks like he's getting it for free, lucky guy. xD
Regarding the different types of builds, it looks like the Aasimar build is just base buffs from your class with flying. The second and third are bodyguard builds that provide the base buff plus the ability to defend allies that are next to you. All of them are really great builds, but they aren't focused on buffing the party as much as just keeping them alive. I definitely agree that Power Attack is going to be required if you want to keep relevant in combat, but if you want a more buff focused character, here is something I went with:
If your party has a lot of shields, Sisterhood Style is actually pretty fun (if you can get the feats needed for it). Basically, with all the feats required, you can give two people adjacent to you +3 to Will and Ref saves and +3 AC (and +1 to Will/Ref at 20 feet). All-in-all, it'll cost you 6 Feats and Charisma 13 (minimum), so Human is definitely looking optimal here.
Likewise, another good feat to use is Iomedae's Inspiring Sword. Granted, this is a better option for Paladins, it has two levels. The Initial Benefit gives you a Full-Round action to give your allies a +2 Sacred Bonus to all Attacks, Saves, and Skills for 1 round + 1 round per 5 BAB. It's... eh, but Sacred Bonus means it stacks with a lot of things (combined with the above, you're now giving +5 Will/Ref Saves, +2 Fort Saves, +2 Damage, +3 AC, and +2 Skill Checks to adjacent allies). If you can get Dazzling Display, you can get the Advanced Benefits which give you an extra effect on your Standard ad Charge attacks. If you hit with a longsword, you grant all your allies who see your attack a +2 Sacred Bonus - so instead of a Full-Round Action as above, it's whenever you hit with an attack.
Granted that accuracy is more important than damage using Iomedae's feat, I'd consider dropping Power Attack, but ultimately that's just my advice if you choose to take this specific feat.
Racial Options: You'll want to keep Bonus Feat because of how feat hungry this is. Skilled can either buffer a lower INT dump, or you can replace it for Heart of the Fey (for +1 Will/Ref, Low-light Vision, and Perception/Nature class skills).
Level 1: Weapon Focus (Longsword); Bonus Human: Shield Focus; Bonus Class: Shield Wall
Level 3: Sisterhood-Style; Bonus: Bodyguard
Level 5: Sisterhood Rampart
Level 6: Bonus: Sisterhood Dedication
Level 7: Divine Fighting Style (Iomedae's Inspiring Sword)
Level 9: Dazzling Display; Bonus: Precise Strike
Using this build, you can give your allies the following buffs:
- Passive: Sisterhood Style, gives adjacent allies +3 Will/Ref and +1 Will/Ref up to 20 feet.
- Passive: Shield Wall; Gives +3 to adjacent allies [w/ heavy shield]
- Swift Action; Sisterhood Style, give 2 allies the Shield Wall feat until Stance ends.
- Swift Action; Challenge (3/Day), +2 circumstance bonus to melee attacks if you threaten the target.
- Swift Action; For the King (1/Combat), Charisma Mod +1 competence bonus to Attack and Damage Rolls within 30 feet for 1 round.
- Full-Round Action; Inspiring Sword, give allies in 30 ft. a +2 Sacred Bonus to Attacks, Saves, and Skills for 2 rounds.
- Standard Action; Lion's Call, give allies +2 competence bonus and Charisma +1 competence bonus vs Fear for 8 rounds.
So - Assuming 16 Charisma, you'll optimally be giving these bonuses to your adjacent allies:
* +5 Will/Ref Saves
* +2 Fort Saves
* +8 Attack
* +4 Damage
* +3 AC
This build will be better when you can start using your Standard and Charge attacks to trigger your Sacred Bonus for your allies at level 10, but until then using a full-round action every other turn might be your better option. Your Lion's Call does last for 8 rounds.
First-turn in combat will probably be Sisterhood Style + Inspiring Display, followed by Lion's Call and Challenge. Definitely try using For the King every combat, since it's an insane swift action buff.
I apologize if I rambled too much with my post... my first ever character that I played a full campaign with was a supporting character. He was a Paladin rather than a Cavalier, but crafting this concept reminded me a lot of him. I hope you have good luck with your Cavalier! :D
(Edit) Sorry, just noticed this. Can you clarify what you mean by "I'd be using a scarlet rose banner attached to a long spear."? Are you referring to the Scarlet Rose Banner (1-Handed, Counts as Heavy Shield), or are you referring to the Banner of Ancient Kings (2-Handed, can be attached to Long Spear)? Both cost 18,000gp, but they have two very separate builds and buffing abilities.
(Edit 2) Not sure how functional this is, but if you get enough Gold Pieces for both of the Banners, you can use AVR's Shield Brace build to use both banners at the same time! Double Banner Build!
Breaking it down - escape route, shield wall and precise strike are the 1st level bonus teamwork feats I suggested. Escape route enhances mobility, shield wall enhances defence (espec. if anyone else in the party uses shields) and any buffs you have that use shields (like vanguard), and precise strike enhances damage. I only suggested one teamwork feat for the level 9 slot because outflank is going to be the best there - outflank raises the flanking bonus to +4 and gives a free AoO on the flanked target when your partner scores a crit.
The first build was about having the mobility to drop in anywhere and provide those adjacent buffs. The second lets you use bodyguard for AC and reflex saves, and has a mistake in it - it wants one more feat, saving shield to work which is annoying. The third debuffs enemies with dazzling display and is better at fighting than the first two.
TMF's right about the banner you're using being a heavy shield BTW. This changes things - not needing to get shield brace to use weapon + shield would make things easier, and sisterhood style (requires longsword) is indeed pretty good.
Honestly, not sure why I didn't put outflank as the Level 9 bonus feat. That and Improved Outflank are really good with this build. Outflank gives +4 Flanking Bonus and Improved Outflank means you just have to be adjacent to the same enemy. The reason why Improved Outflank is good is that you need to stay adjacent to your allies to give them your Shield Wall and Sisterhood Style bonuses.
But yeah, what AVR said - Outflank and Improved Outflank is awesome. :D
I still say AVR's Shield Brace build is the best build, because you can meme with double banners. xD lol