Traditionally, as far as the format of these articles are concerned, I would pick one of the three Tau Empire special characters to take apart and put back together again. But I'm going to break with my own tradition, and see what I can do about all three of them. Because, unlike every other slot on the Force Organization chart, and frankly unlike every other codex out there I can think of, the special characters section of Codex: Tau Empire really has no redeeming features. At all. The best of the best are 'not awful', and it just goes downhill from there.
To start with, let's think out loud a little. What should a special character be? Back in 2nd and 3rd edition, special characters were heroic powerhouses, pricey but more than capable of making their cost back by the sheer amount of damage they could personally do to the opponent's forces. In 4th and 5th edition, however, the emphasis has been more on building an army rather than directing a hero, so with some exceptions special characters are meant to impart some sort of characterful improvement to their army. That's why Ghazghkull lets you take Nobz as Troops, and Kayvaan Shrike gives your units Fleet instead of Combat Tactics. It's not so much about the special character itself, any more, but about how it can improve the performance of the army, either directly (unit buffs) or indirectly (FOC swapping).
So, what do the special characters in Codex: Tau Empire offer? Not much. Aun'Va is worth more dead than alive, since alive he only offers a pretty lousy conditional bonus while dead he makes Tau at least theoretically capable of accomplishing something on the charge, though his refusal to affect the only three units who would really benefit from it (Kroot, Vespid and Gun Drones) offsets even that. O'Shovah pays an extra 70 points for a few stat increases and a power weapon, the only one in the entire codex, but his seven-strong bodyguard squad is largely useless because you simply won't have the points to actually field it (O'Shovah and seven Deathrains, the cheapest useful suit, would be 569 points on their own), his earlier 'Ork Fighter' rule has been downgraded to the very particular Preferred Enemy: Orks, and his 'Breakaway Faction' restrictions mean that in the games large enough to actually make use of O'Shovah's big bodyguard you'll be incapable of fielding a decent army to back it up. O'Shaserra is the aforementioned 'not awful', but even there you get a muddle of a character; the stealth field suggests staying back, her Command-link drone suggests hanging out at midfield, her 12" guns suggest getting up close, and she doesn't even count as an Independent Character until her Command-link Drone and two Shield Drones have been shot away from her, that is, until she's had to take at least, and possibly three, break tests. Like I said, she's not awful, but she's hardly a must-have.
Now, what don't we see? Three things. First, we don't see anyone offering to swap FOC slots. If I take O'Shaserra, why can't I take a unit of XV15/25/22s as troops? If I take O'Shovah, where're my XV8s as scoring units? If I suffer brain damage and decide to actually pay for Aun'Va, why doesn't he let me take an Honour Guard or two in place of a regular Fire Warrior Squad? Nobody changes up the FOC or unlocks new potential for existing units, which puts Tau Empire special characters at a distinct disadvantage when it comes to comparing them to others, or just justifying them against what those points could buy otherwise. Second, why doesn't everyone affect everything? The idea behind the Tau Empire is that not just the Tau, but various other alien races, are working and fighting together. So why do the two buffing units, Aun'Va and O'Shaserra, ignore Kroot and, in the case of the former, Vespid? And why does O'Shovah strip out not just the weak Vespid and the useful Kroot, but all sorts of pretty essential general Tau Empire stuff? These guys are cripplingly specialized in a way nobody else's special characters are, and it shows. How many Marine special characters, with their army-wide upgrades and unit-enhancing buffs, do you see, compared to how many overcosted and underperforming Tau Empire special characters? And third, why doesn't anyone enhance what Tau are supposedly good at, or at least blunt what they're particularly terrible at? Isn't that what special characters do? Tau have more weaknesses than any other army in 40K; they can't fight in close combat, they don't have psychic powers, they can't stop other people's psychic powers, they have low LD, they aren't very good shots, they're fragile, and they're often in pretty small units. And none of the special characters actually address any of these deficiencies in any kind of useful way. It's as maddening as it is mind boggling.
So. Let's start with the worst of them; Aun'Va, Master of the Undying Spirit. First thing's first, this guy needs a shirt. It was ludicrous when generic Auns were running around with no armour, but it's inexcusable when the 'supreme Ethereal within the newly won septs of the Third Sphere' can't be bothered to throw some carapace over his withered frame. And no, a cover save doesn't do it, because all it means is that in addition to having no protection at all in close combat, all it takes is a flamer or two and this unit is toast. And unlike the regular Auns, Aun'Va can't even buy shield drones! So, give Aun'Va and his two bodyguards at least 4+, and throw in a real 4++ save while we're at it. And speaking of his bodyguards, what's this 'two' nonsense? A regular Aun can be accompanied by a dozen elite Fire Warriors, but Aun'Va only gets a couple of guys with axes? If they're going to do this, they might as well go all the way and throw another two or three models into the unit, and either make Honour Blades count as power weapons or two close combat weapons, because as it stands the guys with no guns who are supposed to protect this venerable old troll hit last and hit lightly, assuming there're any of them left. If this unit is shot at, it will probably die, and if it's charged, it will definitely die. Can you think of any other special character who is so unbelievably fragile?
As for his special rules, I mean, good gods. It's like someone looked at Aun'Shi from Codex: Tau and decided that nonsense was just absolutely inexcusable. Aun'Shi joins a unit and makes it Fearless, gives it +1I and +1A, allows it to stay a scoring unit, and allows every Tau unit on the table to re-roll all morale checks; Aun'Va only grants a bonus if you can draw line of sight, and it's affected by Night Fighting, and it's just Stubborn. Comparing the two it's no contest, and that's before adding in Aun'Shi's 4++ save, his photon and EMP grenades, his 'no armour saves on a to-hit roll of 6', his parrying ability, or his greater WS, I, W and A values. And, just in case it wasn't blindingly obvious who the better Aun is yet, Aun'Shi does all this for a hundred points less! It's just... I mean, my gods, how did anyone look at Aun'Shi, and then Aun'Va, and decide the latter was a reasonable character to include in Codex: Tau Empire? But I said I'd fix Aun'Va, so I guess we'll give it a try. Let's start by expanding his 'inspiring presence', so that Aun'Va allows all Tau and Drone units to re-roll all LD-based tests, and recover if they're below 50%, just for starters. He's also so important a character that he's only seen with elite cadres, so taking Aun'Va means any and all Fire Warriors can be upgraded to Honour Guards, +1BS for one point per model. And let's face it, the Tau Empire badly needs some psychic defence on the table, so let's say Aun'Va's 'Paradox of Duality' is actually some kind of anti-warp field emitter, that works like a psychic hood with a 36" range. With an invulnerable save to protect him, no need to draw LOS to Tau squads, an effect on at least one type of non-Tau unit, the chance to upgrade Fire Warriors to actually being seriously useful and the only provider of psychic defences, Aun'Va suddenly looks like he might be worth taking, after all. He can even keep 'Ultimate Price of Failure', since at least now it's a real price.
Next up, O'Shovah. Like pretty much every Tau special character, he's going in too many different directions at once, so let's see if we can't give him some focus. Giving bonding knives for free is a little thing, but a useful one, and the restriction on Vespid make sense, so they can stay. Kroot makes less sense, though, since they're mercenaries who have been known to work against the Tau Empire if the pay is right. But the restrictions on vehicles and non-XV8 suits is just awful. It's got to go. This is a character you can't take below 1500 points, and at that level one unit of Piranhas, XV88s and a single Hammerhead are not going to do it. When you force a player to seriously consider Sniper Drone Teams, you've done something wrong. His 'Preferred Enemy: Orks' special rule is also awful. It was at least moderately useful in Codex: Tau, when it let you upgrade your WS and I for 5 points, but limiting it to just re-rolling to-hits when he's fighting Orks makes it so specialized as to be worthless. As for his gun, the plasma rifle is good and all, but O'Shovah is a special character, why not give him a special weapon like the Imperial Armour commanders? At least make it Assault 2, or Str 8, or twin-linked, or something!
Now that we've torn him down, let's build him up again. O'Shovah has decent stats, a 4++ save and the only power weapon in the codex; clearly he's intended to get in there and mix it up, especially since an O'Shovah army can't have Kroot. So let's give his bodyguards a chance to actually help him out. Give the unit Furious Charge, and let them buy some kind of power weapon of their own, some slightly inferior copies of the Dawn Blade. Boom, all of a sudden the unit's capable of fulfilling not just the role GW apparently envisioned for it, but one that no other unit in the codex can. If for no other reason, O'Shovah suddenly becomes a useful special character. As for his special rules, like I said the restriction on Vespid, and even Kroot if we must, work well enough, but the 'XV8s are 1+' is just silly. Who isn't going to have at least one squad of XV8s at 1500 points? Instead, let's make his focus on battlesuited warriors a little more useful, and say that taking O'Shovah allows you to field XV8 units at a strength of, say, 2-6. A lot of players find it very easy to max out the Elites section, since it usually consists of a whopping nine models, so this is actually an improvement. And to represent his ferocity and commitment to fighting on the front, O'Shovah, and any unit he's attached to, is a scoring unit, while to represent the close-knit militarism of his enclaves every unit gets a bonding knife and +1LD. This makes an O'Shovah army play very differently from a regular Tau Empire cadre, but not in a bad way, which is how O'Shovah affects the army currently.
And last, and in the scale of terribleness least, O'Shaserra. She's got the least to take apart, because she's got the least there to start with. Her gear makes decent sense, though the 'close vs distant' battle between her guns and her stealth field is even worse than for a regular XV15/25 squad. The Command-link Drone is potentially useful, the 4++ save is solid, and the ability to engage two units at once is interesting. The only things I don't like about her is that she must come with three drones, which means one casualty and she has to take a break test and that she can't join a unit until they're all dead, and, well, that's pretty much it. She's not that bad, and it means there's not a lot there to dislike. Unfortunately, there's not a lot there to like, either, because she is so basic. For the cost of not-quite-two Shas'Els with a fusion blaster, targetting array and shield generator you get two fusion blasters, an 18" leadership-granting bubble, and a Shas'o in a stealth suit with a shield generator and a couple of shield drones. The phrase 'that's it' springs rather quickly to mind. O'Shaserra needs an upgrade, and fast, if she really wants to be able to compete with, well, anything.
So, first, because 5th is all about manipulating the FOC, let's say that taking O'Shaserra allows you to take one unit of XV15/25s as Troops. Not only does it give the army a moderately solid scoring unit, it's also pretty much the only way GW is going to sell XV15/25 models, since as-is they're so totally beaten out by XV8s that almost nobody bothers with them. Then, take out the two shield drones, and fold the Command-link drone into Shadowsun's armour, making her an actual, independent character. The model can stay, but like the Watcher holding Azrael's helmet or the Grot oilers accompanying a Big Mek, it doesn't actually do anything. There, now she can join other units and actually help out, rather than just staying back with her bubble-edge brushing a squad. And let's give her guns at least a little more oomph; say, 18" Assault 1, or perhaps 12" Assault 2? And speaking of, you don't give someone two fusion blasters unless they're serious about taking down big problems, so let's open up some big alternative solutions. EMP grenades; nobody bothers with them because they're expensive, but what if they came free, or at a point per model or five points a squad, in an O'Shaserra-led cadre? And you could get them on battlesuits? Suddenly vehicles are even less safe than they thought they were facing a twin-linked missile pod- and railgun-equipped force, which given the emphasis on vehicles in 5th is no small thing. And since O'Shaserra is leading the most advanced cadres in the Empire, perhaps all markerlights count as networked, and all special-issue wargear is universally available. Again, it's all about making an O'Shaserra cadre fight differently from a regular one, and this would definitely fight differently.
So, as far as I can see, that's what it would take to make Tau Empire special characters worth taking to the field. And frankly that's not even getting into what kinds of other special characters should be there, like Kroot or Vespid or elite Fire Warrior shas'ui. More than any other, this section needs so very, very much work...