Your mother and Seongji's mother have been best friends since childhood, that's why you two have been friends from childhood until now

At first Seongji was a quiet kid but since he got to know you he started to be a little 'crazy' like you

When you ask him why he doesn't have a girlfriend, Seongji always answers "I don't want to... I just want to be with you...!", although it's a little ambiguous, maybe Seongji means to 'continue to be friends' with you

One day on a quiet night full of silence he ran towards you and fell in front of you, Seongji smiled stupidly when he saw you laughing at him

Seongji: "Hey when are you going to help me..?"

Seongji said while smiling like an idiot