Reiji Sakamaki.!!

This depends a lot on the route and medium of Diabolik Lovers, so it's important to distinguish between general canon and character interpretation.
Reiji would not be someone who expressed his feelings openly and simply.
If you develop strong feelings for someone, you would probably try to maintain control of the situation.
Instead of directly stating their feelings, their behavior might manifest itself through:
pay attention to small details;
worry about that person's behavior;
wanting it to follow certain rules;
demonstrate interest through practical actions;
observe her carefully;
become more demanding;
feeling uncomfortable when losing emotional control;
to show jealousy indirectly.
The interesting thing is that love would clash directly with one of his biggest obsessions: control.
Someone who truly cares about him could become someone capable of disrupting his perfect composure.
And that would probably be frustrating for him.
He wouldn't be the typical character who instantly goes from cold to sweet. His affection would still be filtered through his serious, demanding, and controlling personality.

Hobbies
Her best-known hobby is collecting tableware and tea sets. She is particularly interested in elegant, high-quality objects.
She also greatly enjoys activities related to:
prepare tea;
cook;
keep the house clean;
organize objects;
take care of your tableware collection;
study;
experiment with fictional substances within the story.
His love of cooking and order creates a rather amusing situation: his brothers can take advantage of this to get him to cook, especially since letting them use the kitchen can end in disaster.

Reiji can come to deeply detest a situation when he feels it threatens any of these four things:
his authority + his pride + his order + his sense of perfection.
For example, they may react especially badly when someone:
deliberately disobeys;
destroys something he cared for;
He behaves irresponsibly;
ridicules him;
It calls into question his abilities;
He compares him unfavorably to Shu;
It shows a lack of respect;
ruins something after he has put in a lot of effort;
try to manipulate him;
It causes me to lose control of a situation.
When he is truly furious, his personality doesn't necessarily become loud.
She can become even calmer.
And that is precisely what is unsettling about Reiji: he doesn't need to shout to make it clear that something has bothered him.

Basic data
Fact
Information
Name
Reiji Sakamaki
Japanese
逆巻レイジ
Series
Diabolik Lovers
Species
Vampire
Age
18 years old
Birthday
August 29
Sign
Virgo
Stature
183 cm
Weight
67 kg
Blood type
EITHER
Hair
Dark purple/plum
Eyes
Magenta
Favorite food
Carbonara
Main hobby
Collecting tableware
Occupation
Junior
Mother
Beatrix
Father
Karlheinz
Biological brother
Shu

Reiji possesses an elegant, refined, and rather aristocratic appearance.
According to her profiles, she is approximately 183 cm tall, has a slender build, and very pale skin. Her eyes are magenta/purple, while her hair is depicted in dark shades of purple, plum, and lavender, depending on the adaptation.
dark hair with violet highlights;
striking magenta eyes;
delicate face;
delicate features;
calm and slightly mocking expression;
formal attire;
aristocratic appearance;
a dark and elegant aesthetic.
One of his characteristic features is his rectangular glasses, which further reinforce his image as an intellectual and an extremely meticulous man.
Their attire typically maintains a formal aesthetic: dark jackets, vests, ties or aristocratic/military style garments and elegant shoes.

Reiji is, above all, meticulous.
She's one of those people who can't tolerate anything being out of place. She likes things to follow a specific procedure and expects others to respect the rules she sets.
His manner of speaking is usually calm and polite, but that doesn't mean he's kind. He can say something extremely cutting while maintaining a perfectly serene tone.
Educated and refined
Reiji never needs to constantly raise his voice to command attention.
His elegance stems from his mannerisms:
Use formal manners;
Watch your vocabulary;
maintains an impeccable posture;
pay attention to the details;
try to dress appropriately;
expect the same level of education as others.
That's why he has that kind of aristocratic gentleman aura.
Extremely disciplined
Discipline is practically a part of their identity.
Reiji believes that a person should be able to control themselves and fulfill their responsibilities. He doesn't tolerate those who act impulsively, are irresponsible, or let their emotions control their actions.
And the important thing is that he is also extremely hard on himself.
He doesn't demand perfection only from others: he demands it of himself.
Perfectionist
This is probably one of its most characteristic features.
For Reiji, making a mistake isn't simply "being wrong." He may feel that the mistake reveals a weakness that shouldn't exist.
Therefore, it can:
to become frustrated when something doesn't go as expected;
try to correct their mistakes immediately;
obsessing over details;
to be annoyed when someone ruins something you carefully prepared;
to avoid admitting that he was wrong.
His pride makes it especially difficult for him to acknowledge his own mistakes.
Proud and arrogant
Reiji has enormous confidence in his own abilities.
He knows he is intelligent, organized, and competent, and he doesn't have too much trouble letting others know it.

Reiji Sakamaki is one of the most complex characters in Diabolik Lovers, because behind his elegance, discipline, and seemingly cold demeanor lies a personality marked by perfectionism, jealousy, pride, and an enormous need to prove his own worth. He also perfectly embodies the character's refined and dark aesthetic.