Semimaru has a one-sided crush on Natsu, however he shows his affection by constantly bullying her. So far only Arashi appears to have figured out Semimaru's feelings for Natsu.
His mother was a night worker, as a result, the parents of his peers often gossiped about his family, leading to Semimaru being bullied as a child. His mother hardly ever cooked, but once in a while, she would make him delicious stew.
Semimaru doesn't know his real father because his mother refused to divulge any information regarding his father.