The Matter of Tukko's AgeEdit
Paarfi has hinted (in Sethra Lavode) that Tukko is the author of the Book of the Seven Wizards, and has Tukko calling Sethra Lavode "Young One" -- this despite the fact that when Vlad asks Sethra how old Tukko is in Issola, she replies "Younger than I am". According to Paarfi, Tukko is the one who recognizes Morrolan e'Drien from the prophecy, and also is the one who clarifies what Sethra's focus should be in the upcoming struggle with Kâna. Paarfi depicts Tukko's behavior in private with Sethra Lavode as less formal than that of master and servant. Tukko's use of the term "Young One" is less than entirely respectful, regardless of the epithet's truth. He even presumes to dictate Sethra's choice of reading material at times. Ultimately we don't know if Paarfi is authoritative on these points, or even how he might know.
At some point in far prehistory, Tukko might have been Sethra's mentor or tutor. This might explain his use of "Young One" even if she is in fact older, as well as his occasional patronizing attitude. Another hint in this direction is that Paarfi writes that it is Tukko who is tasked with teaching Morrolan on how to read the Dragaeran language, which might suggest a past as a teacher of some sort.
Tukko has definitively claimed that Sethra is younger than he is, in direct contradiction to Sethra's own assertion on the matter. Presumably, there is some way, in his mind at least, in which this interpretation is literally true. One possibility is simply reference to Sethra's physical agelessness — since she looks young while he appears decrepit, he insists on using his greater physical age as the metric by which to judge.
Alternatively, Tukko might be considering Sethra's return from the Paths of the Dead as a "rebirth", and therefore asserting that his continual existence for longer than that period gives him precedence in age.
Additionally, as with all situations where the manipulation of time is a distinct possibility — either traveling to the past or future directly, or traveling to a universe where time flows at a different rate — the possibility that both Sethra and Tukko are indeed younger than each other, by different ways of looking at personal timelines, should not be discounted.
Finally, it might be that Tukko is something other than an ordinary Dragaeran, with a vastly longer timeline. See below.
Avatar of Dzur MountainEdit
Tukko's full title "Dri'Chazik a Tukknaro Dzur" definitely doesn't sound like the name of a typical Teckla servant. The word "Dzur" might imply that he is a member of (or founder of, or rebel from, or fond of) the House of Dzur Or has eyes or ears like a Dzur (animal). It might also indicate that Tukko is, in some way, Dzur Mountain itself, or a living avatar of the mountain. If so, the discrepancy in his age might be due to different perspectives on his existence. Sethra's might measure his age as the duration of the avatar's existence, while from Tukko's perspective he would view his age as that of the mountain. Therefore, first Dzur Mountain came into being, then Sethra Lavode was born, then Tukko appeared to her. Additionally, Tukko's soul, for want of a better word, might be serially reincarnated in a Dragaeran who ages normally, which still could yield a situation where each individual incarnation is younger than Sethra's (presumably) permanent incarnation, yet the soul that incarnates is older than her.
A Demon, or Demon-likeEdit
Another possibility: According to Paarfi, Tukko grumbles that the gods are always helpless when the crisis comes. Sethra points out to Tukko that the gods are, in their ability to act against the Jenoine, "more tightly bound than you (Tukko)." Tukko primly objects to discussion of "[his] own condition", even though Sethra thinks that discussing it might be constructive.
From this it could be speculated that Tukko himself is a very powerful being (powerful enough to act against the Jenoine) who is, for now at least, "bound" by some conditions or agreement or sorcery. (The only "binding" on the gods we know of is that they may not "interfere in the affairs of the Great Houses"). That is, Tukko might be a demon, a god, or some other powerful sentient. Sethra seems to believe that the time will come when his "condition" can be improved, though not right now.
This doesn't necessarily conflict with his being the avatar of Dzur Mountain itself.
Knowledge of Sethra's Alter EgoEdit
Though Sethra says in Orca that nobody but Vlad has deduced her secret identity, it seems likely that Tukko is aware of it. Sethra would have to come and go as both identities back to her home in Dzur Mountain, (as she does in Dzur), and Tukko is always to be found somewhere around. He's also very sharp.
Of course, if Tukko is the avatar of Dzur Mountain, he may even be a key factor in allowing Sethra to leave the mountain (as Kiera) from time to time. As such, Tukko would not only be aware of Kiera/Sethra, but actively involved in maintaining her ability to manifest as both. Of course, given how closely Sethra is linked to Dzur Mountain, she may consider Tukko to simply be another aspect of herself--making her statements to Vlad ("Who else knows about Kiera?" "No one.") essentially true.