Green is one of Blue's friends currently attending preschool in the class of Miss Marigold. She is not able to speak but can manage to create sounds such as pants and barks. She is a puppy who loves to knock over block towers, as proven in Blue's Sad Day (which was the answer to Blue's Clues). Green is one of Blue's friends from school. She is friends with each one of her classmates. Green attends preschool in Miss Marigold's class, the only known class in the school building. Her best friends include Blue, Magenta, Purple Kangaroo, and Orange Kitten. Her favorite color is brown which is proven in "Blue Takes You to School."
Green looks just like Blue and Magenta, except for the fact that she is mint green with darker green spots, and her ears are much smaller in design than Blue or Magenta’s ears. She has a dark green nose and two white eyes with black pupils. Two small teeth are shown sticking out from inside her mouth.
In "Blue's Sad Day", Green was once referred to a "he". But, throughout the series, Steve refers to Green as she. It's a common misconception, due to her deeper masculine sounding barks.
In Colors Everywhere!, Joe also calls Green a "she". It is also unknown why she is female with a male actor, thus making her barks masculine than Magenta’s and Blue’s. Although, it's probable that Green Puppy could be a tomboy. It's also possible they couldn't find a female voice actress for Green who could bark.
In Blue's Big Musical, Green and Magenta were showing their pawprints asking Steve if he has found the third clue yet.
According to the official FAQ, it is confirmed that Green Puppy is, in fact, a girl.
Green Puppy has not appeared in any reboot episodes yet, and it is unknown if she will appear again (it is unlikely).
However she has been mention in a video from the Blue's Clues & You Youtube Channel which retells Blue's Big Holiday so we may have some hope that she will appear in the reboot.