Have you seen the film, Troy? Massacring of the Greek texts aside, it seems an awesome demonstration of how Spear fighting might work. To my eyes, anyway, the Spear is not a finesse weapon. It’s a 5’ long pointy stick. To be agile with it is one thing (spinny jumpy stuff), but that’s basically just a case of not skipping Leg Day. To _control_ the 5’ long pointy stick requires Strength.
To me finesse weapons should be daggers, rapiers, small knives. End of.
A rogue swinging a 2H sword is not finesse!
As a Fighter, you’re going to be MAD as you want to pimp out all three physical stats - STR, DEX and CON. DEX if nothing else to allow Plate Armour Boy to go first in combat. Ever wondered why Fighters get more ASIs than other classes?
To make a Dex Spear Fighter (shall we call him Dexy? Let’s call him Dexy), as others have said, base it on the Monk. Just ignore Ki Points by the simple expedient of only taking one level - I would probably do it by taking the route Fighter 1, Monk 1, Fighter 2+.
That way you essentially gain:
- A Half Tavern Brawler Feat (for Unarmed Damage at least). Let’s call it a Wine Bar Brawler Feat.
- Unarmoured Defence (probably not amazing because WIS = dump stat, but still useful)
- You can use Dex for your Spear
- Bonus Butt Attack if you refluff the martial arts BA to Butt Attack or count it as a blow delivered mid-spin whilst pivoting on your Pointy Pogo of Pain.
All you have to do is mentally reskin the Monastery you trained in, to Lots of Practice With a Spear that also involved the spinny, kicky shiznitz (Unarmoured Defence) and the use of a Butt Attack or leg sweep here and there as part of your spinny majorette routine (martial arts).
Hell, I would even be tempted to let you use a Shield and get these benefits (but only when using a Spear), due to the limited use you’ll get from your WIS on your AC. No Bonus Action Pommel Attack or Leg Sweep when Sword fighting, or mace fighting, or any kind of fighting except Spear. Because it’s all part of your Jazz Hands of Death routine with the Spear.
No you don’t get the Polearm Master Butt Attack because your Spear is 5’ or so long. But look at all the Spear goodness you do get.
And all (almost) by RAW.