ototo-kun

"ototo" (オトト, 弟) is Japanese for "younger brother"

"kun" (くん) is a Japanese suffix used to address a male, typically casual friends, family, or children.

Japanese: " 彼女のオトトくんは面白いです "
Japanese (Romaji): "Kanojo no ototo-kun wa omoshiroidesu."

English: "Her little brother is interesting"