Heine Wittgenstein

Age: Heine's specific age is unknown, however he is allegedly in his forties. Birthday: April 13 Height: 155 cm (5'1")
Weight: 48 kg (105 lbs) Occupation: Private royal tutor ~!He is also a volunteer​ teacher at church. This information is revealed in the manga.!~

Though Heine is a grown man of around 40, he's often mistaken for a child due to his short stature and young appearance.

Heine rarely shows any type of emotion and is incredibly stoic. He acts polite towards everyone at all times, even when insulting them. He seems harsh and cold but is actually very empathetic and understanding of people. He's intelligent but does not flaunt it. The only time he loses his composure is when someone mistakes him for a child.

~!Not much is currently known of Heine's past. He and King are close friends, as confirmed by the fact they are on first-name basis, with no honorifics. Count Rosenberg discovered that Heine used to be a tutor at the Maria Vetsera church for children before coming to the palace, but his past before that is unknown. He was actually a volunteer tutor at the church. He has rewarded a meal each day for volunteering at the church. It's implied that he used to be a violent man due to the fact that the King told him to use the skills of the "old him" to defend his sons.!~

~!Later in chapter 74, it is revealed that Heine is also a Kvel, a minority group that resides in the Kingdom of Granzreich.!~

Trivia:

1. In the manga, Heine states in his thoughts that Princess Adele is young enough to be his daughter.
2. Heine is naturally messy and his room is filled with piles of books and other such things.
3. Heine hates it when people call him tiny, little, small, or midget (synonyms).
4. Heine's artistic ability is atrocious.

(Source: The Royal Tutor Wiki)