Based on a storyline by Leatherhead creator Ryan Brown, Dr. Xeinos (often called Dr. X) is an Utrom scientist that has close ties with Leatherhead, who was caught in the blast of the TCRI building in the Image and Mirage comics. She is the only character who debuted in the non-canon (by Mirage Studios standards) Image comics to make it into the Mirage comics. She was left behind on Earth after the Utroms left to their home planet, and found and befriended Leatherhead who nursed him back to health. She also played chess with Splinter. Dr. X met Leonardo and Michaelangelo prior to the testing of Leatherhead's second Transmat Device that led to a second Triceraton invasion (TMNT Vol. 3, #19). Dr. X presumably helped Leatherhead construct his two Transmat Devices, though was not seen when Leatherhead recruited members of the Foot and the Turtles to finish up the first Transmat Device he created (TMNT Vol. 1, #45, TMNT Vol. 3, #19). Dr. X, with Donatello's help, constructs an android exoskeleton for herself (TMNT Vol. 3, #23). Dr. X later began making appearances in Tales of the TMNT vol. 2, where she is often seeing accompanying Leatherhead. In the short story Threads, she was revealed to be the one who found Leatherhead in the sewer when he was still a normal alligator.
The Classical Greek word ξεῖνος (xeînos) is a poetic form of the word ξένος (xénos), meaning "foreigner" or "alien." In the poetic sense, it can mean "foreign friend" or "alien friend." This is fitting of Dr. Xeinos, who continues to live on Earth as Leatherhead's friend even after her fellow Utroms at TCRI have returned to their homeworld.