Lucifer is a quite unique case as it is quite common that when humans obtain powers beyond humanity by sacrificing a part of their humanity they completely overwhelmed by their new instincts and feelings that drown out the human part of their minds. He was the opposite case where a non-human overnatural being obtained humanly feelings and emotions. In his instance the personality, feelings and knowledge of Zarathustra manifested themselves within Lucifer which created mental problems as similarly to how the human mind isn't supposed to process overnatural experiences the mentality of an angel despite similarities is fundamentally different. As such these were alien emotions and he was unable at first to process them so that he had an internal fight of conflicting feelings.
Zarathustra's humanity fought with the indigenous angelic mind of Lucifer but in a key moment the two sides were united through the same goal. Upon this moment they merged together and formed the Lucifer today.
He felt the need to free the World from an overlord-like Div tyrant so that he was ready to wage a full war against his Father. This goal became a fixed point so that he over time became ruthless and partially took on himself some tyrannical aspects as his anger began to collect and boil within him.
One of the parts that merged is Zarathustra's zeal to free the people from their oppressors the Daevas and Lucifer's overhumanly pride in that he developed an immense dedication and fixation on to become a Yazata himself and usurp the tyrant.
Over time after the Fall he started to from the Earth the underworld try to warn and help humans against the oppressions of God. So that over time his anger boiled and he started to draw on radical and extreme methods to save them.
However from time to time he also tried to repair the influence of the other older gods and used the syncretic powers that the humans gave him to strengthen the ties of the people with their old gods by the initiation of old rites and ceremonies.
At the same time he tried to save the so called "Saints", the heroes of his Father in that they became the sacrifices for him.
Beside that he was through the accusations of the humans and their hatred quite hurt as he only wanted to guide them to the light of wisdom.
He however has still shards of his former kindness as well as inheriting parts of Zarathustra's kind heart. Although the moments where it shines through the darkness and unholy light of his are rather rare.
"You are being used by your brutal lord. He has never showed thanks to you or your family my maiden, or that he took the hard earned fortune of your townsmen so why do you not free them and yourself?" --Example of his whispers to a person in distress.
Also should it be mentioned that he still has moments where one part of him has the active role like he reacts on the name Zarathustra as well he displays a somewhat easygoing side where if should one see him who knew Zarathustra directly they would attest a certain similarity.
He has also a fondness for humanity as through Zarathustra he saw their potential and held deep affection for it. So he want to save and protect such individuals that have the potential against the grasp of the poisonous dogmatists. On the contrary he holds nothing but scorn for the ignorant and dogmatists as he sees them as obstacles in the development of humanity. However he admits that their crime they cause in his Father's name also benefits himself and gives him more power.
As Servant:
At first should it be mentioned that his personality may depend on which Class thus which aspects are emphasized.
In this Class he possesses quite much of his personality without the taint of
Innocent Monster. He is quite pleased to be summoned in a "complete form" as supposed to what many of his "Satanists" had tried to call. And one with rather much freedom for himself. As he has watched the progress of humanity he is perhaps the most modernity accustomed Servant. So it is no wonder that he enjoys the hedonistic pleasures where he likes to eat and drink the most exclusive food and beverages.
His views on other Servants are highly varying and perhaps even slightly paradoxical. As expected he sees in the "Saints" just a bunch of marionettes that danced in his father's schemes and plays rather than truly praiseworthy humans. Instead should he encounter one he is fond in to destroy their belief in their so called "holy missions". He likes to elucidate the true nature of their God and take pure sadistic joy in their mental breakdowns before trying to recruit them. Should they still blindly follow God he will take the greatest pleasure in to destroy a sheep, no, a cattle to say it in his words and take them towards Tartaros. In this moments he displays sheer delight. Towards Pagan Servants he on the other side holds a much more respectful and honorable side as he thinks that they are in contrast to the worshippers of his Father living with the truth of the world in their hearts as well as in touch with the world and non-human lifeforms.
In these cases he appears like a different person as he likes to hear about their beliefs and faiths and will also respectfully listen to their opinions. As well he will engage in a fair fight without the usage of underhanded methods (and believe me he has many at his disposal). Also he is even curious to hear about their wishes they want to realize and perhaps even willing to assist them.
His opinion on Divine Spirit Servants is mostly at least respectful while judging them in their "World-management" and treatment of humans. Then he adjust his approach on them as he categorizes them in either Div/Daeva or Yazata. In short- Daeva: object of extermination which he treats with the same hatred and grudge as his Father so that he will ensure that they won't fulfill their desires. Towards Yazatas he is on the contrary respectful and polite but still does not hold back in a fight against them. However he will enjoy it to go all out and if he is demanded.
Towards modern Servant like engineers or inventors he is rather curious but he thinks that the weapon technology has gotten too dangerous so he has no hesitations to break into a gun shop or weapon industry area and destroy it completely.
Towards Greek or (non-Abrahamic) Roman scholars or philosophers he as great respect, applauding their achievements as they more than any engineer or inventor have allowed their mankind to develop. So he loathes the Christian and Islamic havocs for their needless destructions as this crime is inexcusable so should he encounter such a individual he will torment them to death without any mercy.
As in the modern world many humans are fighting for the nature and care for animals he greatly appreciates those men or women and should he see their activities he would subtly help them as well as crush their enemies.
Demeanor towards Master:
His demeanor greatly depends on the nature and affiliation towards his Master. So it is virtually assured that should they be a Christian, Jew or Muslim he will at first be unwilling to follow their commands. Really he is a quite difficult to handle Servant as he will take orders as simple advise more often than not.
In the first mentioned case he will try his best to tell them the truth and bring them back to the rightful path. Then should they follow his teachings and elucidation he will act towards them much more benevolently and establish perhaps a friendship or more.
Should they not he will keep them around just for their Mana but his betrayal is then just a matter of when as how. Should his Master be one of Pagan faith he will demonstrate a genuine friendly manner and will treat them with certain politeness. At any case he sees his Master as inferior and more as simple "Summoner" which he rewards then a true partner or contractor. So he will be quite prideful and unwilling to play the pawn in their relation.
Should he though find a personal liking he will see them more as good acquaintance or if their bond is greater and deeper as equal friend which he treats loyally and with care and fondness. However it is not easy to reach this state.
Relationships and other Dialogues:
Emperor Julian:
"To think they called you an Apostate. No, you were the only heretical ruler that went back to the right path. I wished you had success but I am curious if we see a Lostbelt where your ambitions did succeed?"
Tugarin:"What a shame you are a Warrior that doesnt fight unneccesary fights , holds loyality to your YAzata,protecting the innocent and even showed mercy tto the wild beasts , you were a True Saint not all These Imposters."
Gabriel:
"Can it be dear sis that the reason for you to have a thing for weak but strong-willed individuals is that you fear a strong and strong-willed person would end like me? *grins from one ear to the other* As I remember still how you adoringly followed me and wanted that I teach you in combat or give you hints and tips. Ah, what sweet memories~"
Sandalphon:
"You're as lively as always or perhaps even more my little brother. Why did you have to fight at the other side? Can't you see I did it for the world as well as us!?! You dumb lil brother. *sighs* But okay, I am relieved that we are again in a sense on the same side. Would you be willing to a chariot race for old time's sake?"
Metatron:
"Lil sis you should be in possession of all knowledge and still you mistrust me. Believe me had you seen Father's reaction you would believe me. Just so you know to fight against you was very painful. Had you all supported me so many of us wouldn't have to die. But I hope that we can at least a little bit have a relationship as in our youth."
European Pagan gods:
"I saw through my two lives more gods of various worlds than I could count. In contrast to how the gods of Iran were sharply separable in that most were selfish tyrants and only relatively few were truly benevolent, you are much different. Although unmistakably Gods you are much more humane than them as you have like the humans a good and evil side that you possess. You are more Yazata would I judge. Those Men that betrayed you were pure fools."
Pagans that fought against the Christianization:
"You were right, you did nothing wrong. You defended your Yazata's honor and freedom of your peoples. You have my greatest respect brave warriors. Fear not I am from now on your guardian in their stead."
Saints:
"You imbecilic fools. You have no idea what fools you are putting like even the men of today your hope in a tyrant beyond all reason."
Jeanne D'arc:
"The prime example of what happens to a "Saint". Ridiculous, after all that's happened haven't you realized that your valued God played with your country and the others just for his amusement? What had you for this, what was your reward?"
Jeanne D'Arc Alter:
"Be greeted dear daughter on the right side. I'll show you the truth and will make you a champion Dragon Witch of France. Let us shake their ground."