Protectors are a defensive support class that largely focus on damage reduction, but also have two options for tanking. They also have options for hitting things and healing party members, but those jobs are better taken care of by other party members, especially on Expert. You can get through the EOU main game on any difficulty without a Protector, no question, but it's going to be a far more difficult affair--again, especially on Expert.

Stats

(Full stat table)

Common Passives: DEF Up, HP Up, TP Up
Gathering Skill: Take

Protectors have the stats you'd expect out of a defensive support--high HP, high VIT, semi-low TP, low AGI. Their STR, TEC, and LUC are all thoroughly in the middle-of-the-road category. Don't expect them to make fantastic damage dealers with both their STR and TEC stats--although having okay TEC does mean they won't get instantly annihilated by elemental damage and do have an okay amount of disable resistance when combined with their LUC.


Shield Mastery


Reduces physical damage taken when a shield is equipped.
Hey! Have this on anyone you're using Grimoire Stones to equip shields on. It's good.

Front Guard / Rear Guard


Reduces physical damage taken by a row for one turn. Front Guard reduces damage for the front row, Rear Guard reduces damage for the back row. Has priority at all levels.
Front Guard is the core Protector skill, I would argue, and you'll be spamming it about 90% of the time. It's not as good as other incarnations of the skill--only 40% reduction at rank 10--but them's the breaks.

Rear Guard is thoroughly useless, since the back row already gets halved damage from melee attacks--which basically every physical attack is. Ignore it.
Front Guard is your early-midgame bread and butter. It's not fantastic, but it can certainly make the difference between life and death for someone like a Dark Hunter or a Ronin.

Rear Guard doesn't really have any applications unless your back row is already injured, and frankly that probably won't save them because of the low modifiers. If you need to keep your back row up and running desperately, switch your Protector to the back row and use Parry instead. Or Bravery's Gift.

Defender


Increases all party members' physical defense for a set amount of turns. Has a 150% speed modifier at all levels.
Your Protector's time is better spent elsewhere, rather than casting a defensive buff.
This has some value as a skill to put on a Grimoire Stone for a support role to use. 25% at Lv10 is fairly decent and could potentially allow for riskier behavior if you choose to use more defensive options.

Fire Wall / Ice Wall / Volt Wall


From levels 1 to 4, reduces damage taken from one type of elemental attack for one turn. From levels 5 to 9, nullifies damage taken from one type of elemental attack for one turn. From levels 10 to 15, nullifies damage taken from one type of elemental attack, and restores a percentage of the damage taken to whoever was going to be hit for one turn. Fire Wall affects fire attacks, Ice Wall affects ice attacks, and Volt Wall affects volt attacks. At all levels, the elemental Walls will only activate once on each party member for their duration.
The elemental Walls have a dubious distinction of being terrible skills in theory, and yet still complete necessities. These things were already of dubious usefulness on their own in past games aside from when you knew an elemental attack was coming, and now in EOU they're even more situational. Like I said though, you're still gonna need all of them at level 5 or higher eventually, because there's no shortage of elemental attacks that will wipe our your party.
These are all almost a necessity at separate points in the game if you're running a Protector. Level them. Generally keeping them at Lv.4 for most of the game is worthwhile, but as you gain more levels it can be safer to move to nullification levels as your Protector gains more TP.

Shield Smite


Deals melee STR-based bash damage to one enemy. Attempts to bind the arms of hit targets. Uses triple the value of the user's equipped shield's DEF value as a replacement for weapon ATK value. Has a 120% speed modifier and a base accuracy of 120% at all levels.
I don't hate Shield Smite in theory, as far as giving the tank class a method of attacking goes, but it just doesn't offer enough compared to having your Protector continue to spam Front Guard, especially on higher difficulties.
On the other hand, some things in Expert are so dangerous that your Protector will get more out of trying to land an arm bind or getting just a bit more damage in to take out a critical threat before it blows you away. This can also serve as a Grimoire Stone skill for a Landsknecht or a Ronin in a pinch if you need more sources for arm binding. It's actually got a pretty damn good damage modifier (higher than Raging Edge), Protector just doesn't have the STR to make significant use of it.

Shield Rush


Deals melee STR-based bash damage to all enemies. Reduces hit targets' physical attack for 4 turns at all levels. Uses triple the value of the user's equipped shield's DEF value as a replacement for weapon ATK value. Has an 80% speed modifier and a base accuracy of 99% at all levels.
See my notes above on Shield Smite.
This can similarly help in a pinch when you're facing an enemy (or multiple enemies, in which case this is even more appealing) with significant amounts of AoE damage that can hit anyone in your party. The damage reduction isn't something to be disregarded, and the damage modifier isn't that bad for an all-target skill too. Landsknecht and Ronin can also make use of this skill for extra damage if needed. However, this and Shield Smite both have fairly hefty TP costs, so they aren't to be spammend mindlessly. However, Falcon Slash probably wins over this for AoE elimination for sheer destructive potential unless there's a large number of small enemies.

Bravery's Gift


The user redirects all damage dealt to any party members to themself for one turn. Increases the user's max HP for one turn. Has priority at all levels.
Bravery's Gift is a bit of a bizarre addition to the Protector skillset, since it's a reliable tanking skill, while Protectors and the classes descended from them (Hoplites and Dragoons) usually focus on damage reduction with Provoke for semi-unreliable tanking of physical skills. The nature of it plus its TP cost means it's not something to spam, but it's still worth maxing out at least by the end of the game. Unless you're about to eat an all-party attack, chances are good your Protector can survive it while taking heat of the rest of the party.
This is a very decisive but dangerous move in the Protector's skillset. It costs a lot of TP and it can very easily get your Protector annihilated in short order. As such, I wouldn't consider it very valuable for most of the game. However, once you've gotten some points in Protector's passive skills for personal survival, this skill becomes a force to be reckoned with. It can be useful earlier in the game with a Boost to increase the HP gain if you know precisely where to use it.

Provoke


Increases the user's physical defense and chance of being targeted by enemies for a set amount of turns. Has a 500% speed modifier at all levels.
I normally look down on Provoke and skills like it that aren't for dodge-tanking, and EOU is no exception, especially given that I have no idea what the base and non-base values actually correspond to. It's just... In random encounters, you are good 95% of the time just through use of Front Guard, and in boss fights, Provoke's only really good for single-target or random-target skills, neither of which are particularly common.
I've never been a fan of Provoke skills in the series, especially when they don't have an auto-activation passive or aren't a targeted provocation effect from someone else (for example, Otori in EO3) which means they take a turn of economy from your tank to effectively do nothing. Also, more often than not enemies are either not dangerous enough to warrant Provoking, are dangerous in a way that Provoke doesn't matter, or are dangerous in a way that Provoke will quite easily get your Protector pasted in short order. I wouldn't really suggest putting points into this.

Parry


Attempts to nullify physical attacks directed at the user's row for one turn. Each time an attack is blocked by Parry, the chance of it nullifying an attack again on that turn is reduced. Parry has a limit on how many times it can activate per turn. Has an 800% speed modifier at all levels.
Parry's only worth using if it's level 10 and you have Boost on. Otherwise, it's just a weird higher risk-and-reward version of Front Guard.
This tapers off really hard, but I suppose that's due to the proc being total negation. It takes a long time to get running, but I can imagine a handful of sticky situations in the lategame that this skill could definitely get your front row out of. The same can't be said of the back row, however...

Fortify


Increases the user's physical defense for a set amount of turns. Has a 200% speed modifier at all levels.
Designed to be used with Provoke, and therefore suffers from my opinions on Provoke as well.
This can, however, be applied as a pre-emptive for Bravery's Gift. On the other hand, it also requires enough foresight to actually set up on the turn beforehand, and in a climate like EOU's where rotations are somewhat unpredictable this can potentially be disastrous and have someone get killed in the meantime.

Aegis


Gives the user a chance to survive fatal damage at 1 HP. Can only activate once per turn.
Absolutely invaluable if your Protector is going to be using Bravery's Gift with any sort of semi-regularity. A 30% chance to survive fatal damage, which, in my experience, often comes at the very end of an enemy's turn, is worth the skill points.
This is a decent passive to have for Bravery's Gift, but if you bite off more than you can chew it could easily end up being negated.

En Garde


Gives the user a chance to halve physical damage directed at them.
En Garde doesn't really have the chance numbers needed for me to recommend it wholeheartedly, but it's something to drop points into once you have Front Guard, the Walls, Bravery's Gift, and Aegis at good levels.
I find this passive in particular to be more valuable than Aegis, mostly because it can actually roll more than once which can save your Protecor's life when using Bravery's Gift when tanking something very heavy, which Aegis can't quite do as well.

Safe Passage


Can only be used in the Labyrinth. Reduces damage taken from damage tiles for a set amount of steps.
Nah.
What's a Guard Sole?

Flee


The user consumes a percentage of their current HP to attempt to escape from the current battle. If successful, Flee will place the party at the location they entered the floor from.
Double nah.
Honestly, it'd be nice to have this at Lv.10 on a Grimoire Stone because EOU's escape rates are the pits and some encounters can really use an emergency escape. The only problem is actually, yknow, putting in the SP and getting a Stone with it.

Cure


Restores one party member's HP.
Why are you having your damage reduction support cast healing skills?!
It's... fast, I guess? It's exactly the same skill as Medic's Cure so there's not even really a reason to throw it on a Grimoire Stone with Protector as the source. Protector's got moderately decent TEC, but they've got better things to be doing.

Party Cure


Restores all party members' HP. Has an 80% speed modifier at all levels.
The same but amplified?!
Same as above, but this time it's for a skill I don't actually like (Salve)! Just get Salve on your Medic instead, or use something better like Delayed Heal or chuck out some Somas with a Survivalist.

Prayer


Restores all party members' HP and TP after a battle. The HP restoration amount is based on a static amount plus a percentage of the party member's max HP.
Prayer requires a fairly steep skill point investment, and doesn't provide much value in return.
Damage trading with enemies is way too big for HP and TP for how much this skill gives back in return. You could have it on multiple people via Grimoire Stones if you're really determined to have something like this, though, but it's not really worth the effort.