The following tropes of Nikolai Andromalus appear from the website, TV Tropes.
Some of the tropes contain spoilers.......so you been warned.
Achey Scars: The huge scars on his forehead that stretches throughout his face were received from his victorious, and yet bitter fight against 666 (Trihexa). Nikolai takes a hand to his scar and squints his eyes as if in pain, implying that the beast's power left a strong impression to
Badass Family: Considering he's the last living member of infamous, extinct household who were considered as royal assassins who summarily arrest, convict, sentence, and execute criminals for the Underworld.
Brought Down to Badass: Prior to the previous timeline, Nikolai was considered one of the strongest devils in the world due to his last fight against 666 (Trihexa). After he died, he was reincarnated & time-slipped back into alternative world with his past-life memories, battle experience, demonic powers, weapons, and techniques intact, at the cost of his massive resets over his abilities and powers, where he had to start all over again, in hopes of changing the tragic future he once faced.
Berserk Button: If you've harmed any innocent bystanders, pray that anyone but Nikolai comes to kill you.
Blood Knight: Despite being the calm and level-headed type, Nikolai won't back down from a good fight.
Casting a Shadow: As the last survivor of his clan who uses umbrakinesis with toxic effects, able to use it in versatile ways such as giant blades, projectiles of darkness, and defend himself by absorbing and dissolving enemy attacks.
Dual Wielding: Nikolai fights with two swords in battle rather than the heirloom he has stored within his spatial storage.
Evil Weapon: Nikolai's secret heirloom, Abyss Spear provides a considerable power boost to increase his battle capabilities, but it has a noticeably darker aura...and try to eat him alive.
Experienced Protagonist: Before his eventual death in his previous timeline, Nikolai was already a great swordsman and war hero and extreme badass prior to the start of the series. Even though he has gotten a "re-set" over his powers due to his reincarnation, he manages to tap back into his years of experience in battle whenever he's involved in a serious fight.
Failure Knight: A large part of Nikolai's desire to protect others and the absurd lengths he goes to do it is due to the guilt of believing that he couldn't save or protect anyone during the war with 666 (Trihexa).
Good Scars, Evil Scars:
Good Is Not Soft: Nikolai can be compassionate, understanding, and noble, but make no mistake: if he knows you thrive on the suffering of others, he will consider you a corrupted soul to be eliminated in his goal of cleaning up the world without a second thought.
An Ice Person: With his mother's demon heritage, Nikolai is a highly skilled ice user, able to create many different constructs through his Ice, and able to slay demons and evil beings alike with his Zero Requiem.
Magic Knight: Master Swordsman + Demonic Power + Badass Swords + Magician= Total Badass Knight-Typed Devil for Rating Game.
Magnetic Hero: A core aspect of his personality.
Master Swordsman: He is considered as one of the great swordsmen in the Underworld.
Made of Iron: Nikolai may be physically weak, but his toughness is anything but. He survives getting skewered, slashed up in multiple ways and being hit by overwhelming blunt force. It's lampshaded early on when he and couple of soldiers were hit by magical-powered cannon; the soldiers' hospitalized in the following days, but Nikolai is just fine. When Zelveer questions his durability, Nikolai says it's because of all the mixed heritage he gets from his parents.
No One Could Survive That!: Has on different occasions been heavily wounded, being presumed dead each time, but manage to survive.
Reincarnation: Prior to the fan fiction series, Nikolai was the last one to die on the dying world of future DxD. After he died, he was reincarnated back into the alternative world with his past-life memories intact, in hopes of changing the tragic future he once faced.
Red Baron: In his previous timeline, He is referred to as "The Greatest Swordsman of the Underworld".
Power at a Price: Elphilia notes that every time Nikolai uses the power of Abyss Spear, his sanity starts to lose because of the spear tried to devour its owner. As a result, Nikolai only uses the spear at the emergency and tries to minimize his use of the ability.
Purple Is Powerful: His outfit consists of a lot of purple with a mix of blues, and he is a member of House of Andromalius, a well-known clan that rivals the Great King Bael.
Scars Are Forever: The huge scars on his forehead that stretches throughout his face. He received the scars after his victorious, and yet bitter fight against 666 (Trihexa). Even he was reincarnated, the scar on his forehead remained as a reminder of his mistakes of not able to protect those he cared about.
Shell-Shocked Veteran: Lost many of his friends and lovers during the Second War against 666. While he is not out for vengeance, his grief over those losses is implied a few times and is never played for laughs.
Slasher Smile: Rarely, when he really lets loose.
Super Toughness: In his previous timeline, He can survive from getting cut, skew, and pierced by few dozens to hundreds of holy spears of light and even manage to tank Crom Cruach's immense aura attack and Issei Hyoudou's Diabolos Dragon・G with heavily damages. What makes this so remarkable is that he has been trained for his life to reach that strength
Tranquil Fury: He never gets consumed by anger, but he will not forgive cruelty, whether monstrous or indifferent. Like his Clan, he has the determination to punish the wicked for taking innocent lives.
Weak, but Skilled: Despite being a lack of physical powers and strengths like all of the other people in the Highschool DxD, Nikolai can back it up with an arsenal of swords & assassination techniques, demonic powers, and magic attacks much higher than the main casts of Highschool DxD. He also pulls Determinators out more than even Issei as times, winning by just never giving up and his calm, analytical mind allows him to come up with attacks on the fly. Eventually, he is considered as one of the strongest Devils in the Underworld for his supreme technique-typed fighting style.
Your Soul Is Mine: Nikolai's secret heirloom, Abyss Spear is hungry for souls. In his previous timelines, he had the spear devoured at tens of thousands of demonic beasts left from last fight against 666 (Trihexa).