Related
Despite having the lowest HP in the game, Hope makes for a fairly interesting Sentinel. Like Lightning, he doesn't get Challenge and favors Elude over Mediguard and Steelguard, but Hope also gets the counterattack Abilities, Entrench and Vendetta. He's fun to play around with, but not much use against truly powerful enemies.
Lightning is an interesting Sentinel, relying on the Elude Ability rather than Mediguard or Steelguard. This Ability (the only one she can use besides Provoke) allows her to dodge attacks with a very high rate of success. However, it only works on Physical attacks, making Lightning a sitting duck against Magic-wielding foes. Still, when it works, it's very effective and requires very little healing.
Sazh is all about defense as a Sentinel. That may seem to go without saying, but he doesn't get any evasion Abilities, and while he does get Vendetta he does not get Entrench or the Counter auto Ability. He also doesn't get Mediguard, so if Sazh is your defender, you'll need to keep an eye on his HP. He does have the second-highest HP total in the game, though, and is the only non-primary Sentinel with Challenge.
Snow is the prototypical Sentinel, gaining every Ability except Elude. Snow's job is to stand there and take punishment. While he has the same Ability list as Fang, Snow is a bit better as a Sentinel because of his high HP total and access to Weapons with the Improved Guard and Improved Ward abilities.
Vanille as a Sentinel? Surely not! And truth be told, she's not very good at it. She has Mediguard at least, but not much else, including Challenge. She does have more HP than Lightning or Hope, but both of them are evasion Sentinels anyway. Basically, you don't want Vanille to be your main Sentinel. She's fine in the Tortoise (SEN/SEN/SEN) paradigm, but not much else.
Bonus Type
Damage Reduction