(There's no shortage of historical parallels to this; people who certainly could afford to wear solid armour would decline parts - most often the helmet - or even all of it basically just to show off; and even humble warriors might fight outright naked as a statement of their fearlessness.)
I have never heard nor seen evidence of this 'historical parallel' in which you are mentioning. If one could afford to wear proper armour, they would rarely ever willingly omit to wearing parts of their panoply, especially not something as vital as a helmet, especially not if it is expected that a battle/fight might soon occur.
Unless there was a situation where it was absolutely necessary to refuse the donning of parts of their armour, then they would likely have worn as much armour as is available to them, yet does not imped them from performing a certain role in combat, such as Archers who might wear a brigandine or plackart with jack-chains et cetera and helmets with a clear field of view but with no out-jutting brim (To allow movement of the arm to draw a bow), or where the terrain or location of a region affects the wearing of armour, such as in Ireland where the marshes and bogs often forced the Irish to fight in armour without leg defences (They also often neglected hose).
For example, if a soldier was expecting to perform in a skirmishing/raiding/ambushing role, they would have likely opted to only wear textile garments akin to an Aketon or Jupon as their primary defensive garment besides a helmet, even if they owned or could afford better armour. An aketon or jupon should be enough to protect against most indirect slashes from bladed weapons excepting polearms, whilst not being weighty enough to limit ones performance in mobility and speed. (Though if said Jupon was similar to 'Charles de blois' Jupon, it would probably offer protection against even a blow from a halberd, but definitely not from a thrust, as there is about 1 solid inch of padding throughout portions of his garment)
When people are expecting to fight in a battlefield, there is no excuse for not wearing a helmet, other than said helmet being knocked off in an engagement with the enemy.
The only point at which I know when people '
willingly' declined to wear parts of their armour (With the assumption that people could afford their armour in the first place) occured during the 1500's-1600's, and this is largely due to the widespread adaptation of matchlock arquebuses, calivers and muskets/demi-muskets into armies, and this was likely because 'proofing' the armour at the legs and arms would have been almost impossible (Up to that point, leg and arm armour might only be some ~0.7-1.2mm thick, more than enough to provide adequate protection to most melee weapons, making the armour double this thickness would have doubled the weight). Even during this period, where arm and leg defenses were going out of popularity, there was an increasing development of larger tassets in armour during this period. Armour itself was quickly becoming heavier and heavier, hence why you see mercenaries and soldiers slowly beginning to refuse to wear portions of their leg and arm defences which would not be able to protect them against arquebus fire, they were being too over-burdened. The final death-knell to armour likely arrived at two points, the wider usage of artillery in field-armies, which combined with the wide-spread usage of '
Spanish' muskets, made armour ineffective (These being large muskets that require a forked rest and are pretty-much portable 'wall-guns', firing shot of about 10 to the pound or larger in weight, to velocities of about 400-450 m/s).
Before the 1500-1600's, wherever one could afford to up-armour, they up-armoured, especially in regards to the legs and arms... If not, people would find that if they had to oppose opponents in a field of battle, especially plate-armoured opponents in a field of battle, they would have the tendency to get 'Wisby-ed', which is to say that people with polearms will be making mince-meal of their arms and legs or heads by people not too afraid to get within their range (Given that their armour allows them to take more risks) and absolutely wreck havok.
My point I feel, Is reflected in virtually all assizes relating to military service in the Medieval period. Freeman and burghers who did not own either padded textile defences, spears or helmets, could be exempted from service and fined... and if said Freemen did not own any weapon at all, they were often exempted from being 'Freemen'. If they did not have a helmet or armour, they were exempted from service, and fined. This I feel, is an important point.
The famous Gaesatae for example, perhaps one of the few examples of people fighting naked in history, likely wore helmets. In this case, they didn't really need to wear armour because the coverage of their shields would have been adequate in most situations (By which I mean that most Celtic shields of the period were much like the Italic scutum and could offer protection to much of the body). A helmet, however, is vital.
If you cannot dodge out of the way of an oncoming strike, are not afforded the protection of a shield and you do not have armour, what are you?
You are dead, or at the very least, injured.
If you were expecting to fight a battle and willingly decided to not wear your helmet, did not wear every part of your panoply and instead opted to fight naked without a shield, then you are almost certainly going to die. And if you die, you will have died Foolishly, its as simple as that.
Even if Sabia wanted to be ostentatious, there is pretty much no excuse for whatever Sabia is she wearing, except for 'This is a porn game... roll with it'.
When nobles want to be ostentatious, even if they are not really expecting to fight, the armour that they commissioned would generally be capable of offering protection and would most likely function as a defensive garment. From the bronze musculata worn by Imperial generals to the gilded and etched surfaces of Greenwich armour, there is no excuse for 'bikini' armour. There certainly isn't much excuse to omit from wearing arm, leg or head protection either, especially when one is able to afford such defensive protective garments.
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Or TL : DR, I highly doubt that there is a historical parallel whereby anyone would willingly refrain from wearing a helmet or parts of their armour without any exceptional circumstance or context for doing so, even if said person could afford the best armour possible. In fact, it seems that Noble warriors seeking to be ostentatious, wear as much armour as is possible, though the armour that would have been worn would likely have been decorated, gilded, etched, blued et cetera. Hell, even if said noble wanted to show off without using armour, they would opt to wear as much clothing as is possible, because the more fabric that a garment uses in its construction means that the wearer has to pay more, and that is very show-offy.
As to people fighting naked, that was very rare. Even in the occasions in which it did occur, Shields were likely used and helmets were probably worn.
Whenever and wherever it is possible, people will willingly wear as much armour as they can afford, so long as they can still function in their given role. They would not shirk off parts their armour for any sake of pretentiousness. If they did, they were being idiots.
Armour is important, especially when one does not have a shield. The helmet might just be the most important armour of all.
Armour was important enough that it's lack thereof could see a freeman fined.
Sabia has no excuses, in my opinion, except for that of 'It's a porn game, roll with it'.
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