Leonhard respects his brother Bruno and sees him as a brilliant person. Because Bruno is very intellegent, Leonhard is amazed by his knowledge.
Leonhard cares about Kai just like his other siblings. When he accidentally offended Kai, he apologized immediately, and it show that he cares a lot.
Leonhard sometimes argues with Licht but rarely. He calls Licht his baby brother and tries to impress him sometimes.
Viktor is his and his brothers and sister's father. He loves his father dearly but is not shown, unlike Kai shows his love to Viktor.
Leonhard loves his sister very much and is very protective of her, as seen in Chapter 15 of the manga. When he heard that Heine accepts to be Adele's "husband", he didn't approved and took Heine's acceptance seriously, while Heine is just doing it to lure Adele out of her room.
Leonhard finds Eins to be scary and mean because Eins called him a dunce or stupid. They are not close.
At first, Leonhard did not like Heine because he was a teacher and believes that teachers are evil. After the interview with Heine, he starts to like him because of how Heine didn't laugh at his journal of regrets. He felt that Heine was a bit different from his previous tutors, and Heine usually gives Leonhard hope that he can be a great king candidate. Leonhard also likes to be praised by Heine, and he acts like he dislikes Heine.