Max would have been the name of Jace's unborn baby. The father was a Manticore technician named Victor. After escaping Donald Lydecker and getting travel papers, a pregnant Jace informed her sister Max Guevara that she was going to name her child after her. Max was presumably born months later.
Jace didn't know the gender of the baby when she left Seattle, but prior to this, Dr. Vertes had informed Max and Logan that it was a girl.
It is unclear whether Max would inherit X5 abilities. In "Hit a Sista Back", Lydecker and Elizabeth Renfro stated that human/X5 hybrids created under laboratory conditions never had, with Tinga's son Case Smith being unique. On the other hand, Dr. Vertes said the baby was "strong, like her mother".