Isabella is a character first introduced in Ghost Smile: 2nd Chance. She has been confirmed to be a major protagonist in the upcoming Ghost Smile 3, which will mark her first role as a playable character.
In Ghost Smile: 2nd Chance, Isabella was Smile's mortal wife, unaware that he was a ghost. She was essentially a two-dimensional character with little personality. In Ghost Smile 3 (which is essentially a reboot of the Ghost Smile franchise), however, judging from what little information has been released about her, she is a musician with an artistic streak who is experimented on by Mythic, who literally fuses her with a Ghostpunk. She manages to escape Mythic's facility and uses her new ghostly powers to assist Smile on his quest to defeat Mythic.
Isabella is generally sarcastic and flippant in dangerous situations. She remains mostly unfazed by the various unnatural things that happen to her.