Highlanders are a support-y damage class, not terribly unlike Landsknechts, but with a more unique and more generically useful skillset, and also a focus on draining HP to use skills. The class is normally exclusive to the Story mode protagonist as far as pure Highlanders go, and you can only change class into Highlander when using New Game+ on a finished Story save--which is ludicrously stupid if you ask me. Such is Etrian Odyssey Untold, though. As far as effectiveness goes, Highlanders have the second-best STR stat of any of the classes, and their skills generally deal high damage anyway, meaning they're very, very effective damage dealers. Their support skills vary in usefulness, but I can imagine uses for most of them.

Stats

(Full stat table)

Common Passives: ATK Up, DEF Up, HP Up
Gathering Skill: Mine

Highlanders have a pretty above-average stat spread. As mentioned above, their STR is the second-highest of all the classes (only 5 below Ronin STR at level 99), while their HP, VIT, and AGI are all above-average. Their TEC and LUC are not spectacular, to put it a bit lightly, and their TP is absolute garbage.


Spear Mastery


Required to learn spear skills. Increases damage dealt with spears.

Long Thrust


Deals ranged STR-based stab damage to one enemy.
Long Thrust is as generic as skills can get, really, but it's a pretty cheap option for damage, and it's probably what your Highlander will want to stick with until they have a higher TP pool, because Spear Assist, the other generic "use this when there's nothing special to do" skill, costs way more TP at basically every level.
Highlander will be running this for a decent while until things like the Cross Thrust combination become more reliable. It's not amazing after that besides for cleaning up small trash, but Highlander has better tools for that anyways. I suppose it could be more reliable for moderate trash where full spread isn't very effective and heavy damage setups are too costly. The ranged property allows for priority elimination as well, which can be nice.

Draining Thrust / Legion Thrust
(Draining Thrust)
(Legion Thrust)

Deals melee STR-based stab damage to the enemy front row. Draining Thrust consumes 20% of the user's current HP upon use. Legion Thrust consumes 15% of all party members' current HP before use. Has an 80% speed modifier and a base accuracy of 108% at all levels.
Some fairly generic encounter-clearing skills. Not that high damage, but they're kinda cheap as far as encounter-clearers go, even when factoring in the HP cost.
These are very efficient small trash clearing skills. Combined with Turning Tide, these skills can keep your HP filled up relatively easy for a lot of fights.

Draining Burst / Legion Burst
(Draining Burst)
(Legion Burst)

Deals melee STR-based stab damage to all enemies. Draining Thrust consumes 40% of the user's current HP upon use. Legion Thrust consumes 40% of all party members' current HP before use. Has an 80% speed modifier and a base accuracy of 108% at all levels.
Eh. Legion Burst is kinda pointless since you're draining nearly half of your party's current HP values for not that much more damage. Draining Burst's dicey even without that, since you're using 5 more TP at level 10 and 20% more current HP for barely any more damage that can hit the back row, though that damage does get halved.
These skills aren't quite as economical, but they can come in handy for enemy formations with nasty backliners.

Head Pierce


Deals melee STR-based stab damage to one enemy. Attempts to instantly kill the target. If the instant death fails, attempts to bind the head of the target. Has an 80% speed modifier at all levels.
This skill had a way cooler name in the JP version that was already just English: Brain Rend.

Oh yeah Head Pierce sucks don't use it.
This costs too much for the proc chance to be used against individual targets. It's mostly a skill for head binding, which Gunner does anyways. It's also useful for the few conditionals that require instant death to make items for classes that Story doesn't use.

Spear Assist


Deals melee STR-based damage to one enemy. If an elemental attack was used before Spear Assist, its damage is doubled, and it deals stab+element damage, where element is the element that was used before it. If not, it only deals the base damage, and only deals stab damage. Has inverse priority at all levels.
This is one of the two later skills that Highlander ends up using. With an exploitable weakness, Spear Assist can do pretty hefty damage without much setup, and it's beastly with Action Boost.

Delayed Charge


Consumes 15% of the user's HP on cast. A set number of turns later (two at levels 1-4, three at levels 5-9, four at levels 10-15), Delayed Charge will activate at the end of the turn, and deal melee STR-based stab damage to the target. Can be activated early with Cross Charge--however, this will reduce Delayed Charge's damage. Has a base accuracy of 178% at all levels.
Delayed Charge exists basically solely to be used with Cross Charge for big burst damage. See below.
Part one of the one-two combo.

Cross Charge


Consumes a percentage of the user's current HP. Deals melee STR-based stab damage to one enemy. If the user has Delayed Charge active, Cross Charge will immediately activate it upon the user taking their turn, effectively using both skills at once. If Cross Charge activates Delayed Charge, Cross Charge's damage is doubled. If Cross Charge activates Delayed Charge, it has 118% base accuracy at all levels.
Delayed Charge plus Cross Charge when Delayed Charge is ready to go off anyway equals big, big burst damage, with a pretty hefty effective damage per turn, especially for the TP cost.
Part two of the one-two combo. These skills executed together are Highlander's bread and butter, and it's typically your modus operandi for most of the game. It's pretty damn strong.

Turning Tide


Restores all party members' HP when the user defeats an enemy.
Blech. Barely restores anything--pass.
It's okay for when you know you can clear wide swaths of enemies while dungeon crawling.

Bloody Offense


Places a buff on one row that increases attack, at the cost of 10% of the party member's HP each time they take an action. Has an 80% speed modifier at all levels.
Bloody Offense is often touted as the best buff in the game--and that's because it is (although it does share the exact same attack increase value at every level as War Cry).
Story's War Cry. The tradeoffs are sort of a pick-your-poison kind of thing, but Bloody Offense is just as effective. A very good attack buff. Just make sure you remember to shove Arthur into the front row before you use it.
Bloodlust


When the user loses HP, they will perform a normal attack (with a damage bonus, from levels 2 to 10) on one enemy.
I mean, once you have all the spear skills you want plus Bloody Offense, there's worse things to spend skill points on than what amounts to a free extra 135% damage when your Highlander casts a lot of their skills or does anything under the effect of Bloody Offense.
Highlander loses HP passively through their skills a lot. This is a pretty good passive, all things considered.

Stigmata


Binds the user's head, arms, and legs. Attempts to copy the user's ailments and binds to one enemy. If the user has immunity to a given bind type, it will not be inflicted. Has an 80% speed modifier at all levels.
Stigmata's absolute trash on Highlanders, but it can be hilariously broken in the hands of a Hexer. Not as broken as in EO2U, though, due to the lack of Creeping Curse.
For the sake of balance, I'm glad that this is normally separated from Hexer by the mode wall.

Battle Instinct


Gives the user a chance to place a buff at the start of battle that nullifies one status ailment on all party members.
a 30% chance at level 10 to give a buff that nullifies one ailment lol
Just go read the freakin book already most of the drinks aren't that amazing anyways

Blood Fortune


Consumes 15% of all party members' current HP to increase their chance of inflicting disables for a set amount of turns. Has an 80% speed modifier at all levels.
This is completely useless for any party that doesn't have a Dark Hunter, Hexer, and a Highlander in it--something that's barely ever going to happen in normal circumstances.
Story's party setup has disables on 4 of the characters? I guess? It's there to help them if you really think you need it.

Allied Bonds


At the end of the turn, restores TP to all party members that had their HP drained by a skill casted by the user.
Way too many skill points for a skill that'll, realistically, only proc on maybe Bloody Offense for most parties.
This is effectively Trickster but slightly better for Highlander, but Highlander isn't running a skill that costs 32 TP. It could help with Action Boost shenanigans, I guess?

Spirit Shield


Consumes 10% of the user's current HP to increase one row of party members' elemental defense for a set amount of turns. Has a 150% speed modifier at all levels.
all those skill points for a row-target elemental defense buff
Story has a freakin' Protector in it. And a Medic. Seriously.

Limitless


Increases the user's physical attack on the next turn. Allows the use of all skills, even if the user does not have the required equipment. Has no speed modifier at all levels.
A Boosted Limitless is a fairly neligible charge skill. Un-Boosted, it's completely pointless. I genuinely can't think of a situation where you could make good use of the "required equipment actually isn't required" thing.
I typically pop this before firing off Cross Charge, although a Charge Grimoire Stone would probably work better than this.

Black Sabbath


Deals ranged STR-based untyped damage to all enemies. The damage is capped at how much total HP the party is missing. Restores all party members' HP, and transfers any ailments and binds enemies have to all party members. Has no speed modifier at all levels. Cannot miss.
i beg your pardon what
These are extremely strange modifiers. I suppose if you could predict high full-party damage you could take advantage of this with a Boost, but EOU's HP totals on enemies are too high in comparison to any given amount of lost HP on a party to make this worthwhile.