If you're looking for manga similar to Code: Leviathan, you might like these titles.
"The chief of the dragon species, born of the first darkness alongside the world's first magic - eleven Imperial dragons opposing man's power of conquest." Weill, whose magician mentor was killed by an imperial dragon, sets off to exact his revenge, but...?
The dragon felt that the worst thing that could happen in the world was to suddenly become a super fierce dragon without knowing anything! He felt bored. He wanted to go snatch a princess and have some fun. As a result, he fell in love with the knight. Then, he should be even more persistent about snatching the princess. The stupid dragon seriously thought so.
Lufiak Duell is the member of a Dragon Clan, but in order to become a true member he has to find a dragon's scale, and he only has one year to do this. Along the way he meets a fairy named Feenie and finds an orb that contains the soul of the Master Dragon. When he meets the dragons they tell him that a plague known as the Dark Force has been released and is slowly infecting the world. Lufiak joins the dragons in their quest to stop the Dark Force.
Raiki, a high school student, transforms into a dragon because he was born with the 'Beast Code DNA' that laid dormant within him and generations of other people for centuries. Raiki now struggles with his new abilities and clashes with others who also possess the Beast Code DNA but plan to use their powers for wrongdoing. Conversely, Raiki's girlfriend Tomoko possesses DNA that instinctively compels her to battle against anyone who has the Beast Code DNA. What is the fate of this couple who are now forced to fight to the death because of their genetic code?
The young lord of a small and weak country saves a dragon beside the sea one day. Since then, his country became prosperous, rich, and powerful. This is the story between a Lord who is a tease that refuses to get married, and a poor dragon who is teased day in and day out.