Masaomi has a very friendly, enthusiastic, and outgoing personality, and is almost always seen fooling around, smiling, making bad jokes, teasing his friends, and hitting on girls. He is shown to care deeply for his friends, having to act as a sort of big brother for Mikado as he adjusts to life in the big city and going to great lengths to protect those who are close to him.  

Source: https://durarara.fandom.com/wiki/Masaomi_Kida