Maromi is an adorable little dog created by Tsukiko Sagi and is Tsukiko's only friend. Maromi is the star of a popular children's show in Japan "Mellow Maromi". Maromi was also the name of Tsukiko's pet dog which was ran over by a vehicle 10 years ago.
This triggered the birth of Lil' Slugger (Shonen Bat). Tsukiko couldn't bring herself to tell her father what happened, so she made up the story of being hit by Lil' Slugger. Maromi is willing to always protect Tsukiko from Lil' Slugger no matter what.
Maromi is pink, cute, and voiced by a female actress, though that does not necessarily mean Maromi is female. The name "Maromi" is a female name, however, so Maromi is probably female.
Maromi is a stuffed dog fashioned by Tsukiko Sagi. It's based off of a dog that Tsukiko owned when she was a child, who died when Tsukiko was attacked by a boy with golden skates and a golden helmet... or so she says.
Now Tsukiko, older, is pressured every day to make a new toy, to back up Maromi and make the business she works for grow ever stronger. When she has nowhere left to run, Tsukiko hits her own leg with a metal pipe, and re-makes the story of Lil' Slugger, the boy who had attacked her ten years ago.
Maromi and Lil' Slugger are the same being. They give false hopes to people, a false escape from the truth, a way to break free from the paranoia.
The truth behind this lovable dog is that when Tsukiko Sagi was younger, she lost her concentration and let Maromi's leash go. When she realized what was going on, Maromi was hit by a car, and killed. Not wanting to get in trouble with her father, she made up the story of Lil' Slugger.
This pink, big-headed, floppy-eared, large-eyed dog has taken the hearts of nearly everyone, spreading faster than most any childhood icon. Also, Maromi appears to be able to talk to her creator, though, this is an illusion in Tsukiko's mind.