Arthus is the leader of the Templars though what exactly he is is a bit of an engima.
Arthus appears as a mid 30s tall man with just below shoulder-length white hair, stark red eyes which are both natural and pale skin. Because of his status, he wears full Templar colours in a mixture of armor and cloth with a sword at his side despite being bad in melee combat.
His true appearance is that of an huge ancient dragon in a mixture of black and white flesh in a constant state of rot and bone somehow still functioning without struggle as if still a being of flesh and blood. His limbs seem hap-hazard in size and structure like many of the bones poking through his hide like they came from other beings he came across possibly during one of his recovery periods.
By all accounts of what he appears to be and everything he can do, he should not exist. His actions are incredibly erratic and make very little sense in the grand scheme of things.
Arthus seemingly suffers a dual personality as how he acts in either form are almost the direct opposite to the other in demure.
In his human form, Arthus strikes as one of those who commands authority which given his position and who he is in charge of is no surprise. He actively tries to keep on good terms with the Healers to the point he was the one who tipped off Sendeni where his sister potentially was likely betraying some of his original heritage. He makes it very clear their targets are only dragons who could be considered dangerous and a threat to everyone if left to run around and is aggressively enforcing of this rule going personally to help check the status of a target and avoid Dragon Lords as much as possible
While perhaps a little terse and certainly not softly spoken, he has been known to randomly help people as he wishes or ordering his Templars to do the same if an area has been hit by something, particularly in the aftermath of a Dragon attack. This in turn ended up giving him a growing following and further unsuspecting recruits to his cause in hopes of continuing the good work he does for the community. He does treat his own well though his anger has been known to show if orders are disregarded.
As his dragon persona however his only goal seems to be obliterating whatever earned his ire by magic, his own body or both should he see fit. His anger seems so blinding he stops caring what gets caught up in the fight seeing others as just more things to be removed no matter what side they happen to be on. His usual intelligence and ability for cunning and even reason seems to go completely out the window for a crazed creature that is clearly suffering in agony meaning it is possible the human state is suppressing it enough to allow him to function almost normally.
In both forms however he can really hold a grudge strong enough to penetrate his scrambled head.
After being beaten pretty badly by Kale (And likely adding another to
his list), he has disappeared entirely with the partial destruction of
the former Templar held city. Given this is the second time he's been
smashed around so much he should be dead, it's likely he's managed to
pull his consciousness somewhere to recover and seethe for a while.
Given the clear age of him in his dragon form it is certainly possible
he was around already the time of the early Lords like Kale though if
that was in his original state or what he is now is unknown. As such,
there are no records of what exactly happened to him to create such a
creature which is clearly not self made nor why a dragon sharing a
similar ability to the Lords themselves.
There is an odd rumour circling around that he was once in contact with a Lord but something was attempted, went badly wrong and he is the result but if there is any truth in that is a big unknown.
His earliest known blatant appearance actually involved Hance from a distant time in the past though the version of Hance would be the current one. Lords from across time were summoned together due to a massive disturbance of world in peril nature which also included the dragon form of Arthus in a highly agitated frenzy state. It's unknown exactly what triggered him but it was enough for rampant destruction and nearly killing the ancestor of Hance's future wife. Ironically this would be what would make Arthus go for him for the memory incident causing the vicious circle between the pair of them to continue.
Given it is hard to guess exactly when the incident happened in relation to the general time line, it is clear Arthus spent at least a small chunk of time recovering from his injuries before going on to set up the Templars about 30 years prior to their second meeting.
Arthus can shift between his dragon form and his human form at will but
it does appear the longer he's a dragon, the more unstable his sanity
becomes until he just loses it and has to be forcibly stopped or sated
by the destruction of the target. It's very possible the pain from the
fact his very being is rotting inside out and the fact he is effectively
immortal that are causing the snaps. The shifting ability makes little
sense as only Lords should be able to do this yet it's very clear Arthus
was a Dragon originally. He has also shown the unique ability of
partial shifts without consequence.
Likely because it's nothing compared to what he puts up with, he has a very high pain tolerance to the point of not noticing some injuries. In turn, his recovery periods seem to mend any injuries he may have sustained in his dragon state no matter how severe. It is very possible it is because his body is going from an original blue print and making use of any resources it can get. Normally fatal injuries taken in his human state may required a sort of hibernation to clear them up again depending on how much resources his body has at the time.
Surprisingly he can actually heal a little which none of the Templar Elite Guard can. It is nowhere near as good as a Healer though nor a green dragon. He primarily does it for just preventing severe injuries getting worse than actually being able to sort them and shows this skill very rarely even on himself.
His magic is very dark in nature and focuses primarily around attacking the mind, inducing pain and is possibly how he's causing the conversion of the Templar Elite Guard into the demon like creatures. It is certainly not dragon based in origin and likely yet another weird quirk on his existence. He has commented dragons are immune to some of his abilities.
He can alter memories in a far less damaging way than he did to Hance fairly easily. He sometimes does to his Templar forces to ward them off certain subjects involving him simply answering "If I don't want you to remember this, you won't".
As a dragon however former, he does share their ability to recognise a Lord on sight. While he does try and act otherwise by either excusing himself or having a frostier outlook than normal so he can avoid as much contact as he can. For whatever reason they do seem to make him feel more edgy than normal despite it would be very difficult for one of them to tell what he is without making it obvious and certainly does not act as many dragons do around one.
He must eat regularly or his human body does start to show physical signs of starvation. This is from his body burning up to keep the appearance and to heal any wounds he gains so lack of sustenance starts to let his dragon self through. Too long without food and he can be prone to violent rages though has little memory of them afterwards. This is likely partly the reason he keeps around people so much, lacking the ability to feel hunger, taste and little smell it would be easy to simply not notice it happening. The Elite Guards try to ensure he does.
He has little involvement with the day to day running of the city, pointing out short term to him is around the ten year mark. He quite happily helped with longer term and often predicted problems before they even arose likely showing his age.
Number 2 on the hate list thanks to the incident in the far past. His grudge ability seems to not associate being stopped in Dragon form as a good thing and instead sees them as a target of revenge.
Number 3 on the hate list for the most recent spat though his ego might have been sated a little bit given how long he did stand up to the oldest Lord before finally beaten. However that incident has more than likely earned ire from a second Lord in the process... Another victim of the timeloop.
Arthus has no issue with her whatsoever. His kidnap of her was more bait upon realising that this time period for himself was soon to be over and turn it into a later advantage for the Templars.
As the Templar leader, Arthus has no quarrel at all with Sendeni and is quite happy to seek or give council should it be required. He'll admit Sendeni is certainly one of the better choices for leadership they could have made and sees them both trying to do the best for the world in their own areas.
As with the Healers he is somewhat contracted to serve the King, a notion he depises and is frequent to ignore all but that he deems necessary. He claims he is a useless man far to reliant on his Geist agents and that he would happily slaughter if not for the short time enjoyment in the chaos leading to longer term anarchy that would be bad for everyone. The fact he did try and have Geists follow Arthus around for a while also did not help matters though their since disappearance has seemingly gone unnoticed.