Xavin is a fictional character in the Marvel Comics award-winning series, Runaways. She is a Super-Skrull in training, created by author Brian K. Vaughan & artist Adrian Alphona, and debuted in Runaways vol. 2 #7.
Xavin is a shapeshifting Super Skrull in training from the Planet Tarnax VII. Being a Skrull, for whom changing shape (and gender) is of no more importance or difficulty than changing the color of your hair, Xavin is sometimes male and sometimes female. They (even the characters become confused over the proper pronouns to use) possess all the powers of the Fantastic Four (though not nearly on the same level as them) and join the team as the result of a bargain made long ago between their parents and Karolina's.
Xavin loves to boast about how they have the most training and military experience of anyone on the team and is happy to lord their Super-Skrull abilities over their teammates, but beneath all the bravado, Xavin is insecure about their abilities, is ashamed of their failure to end the war with Majesdane, and fears that Karolina is only putting up with them out of pity. Likewise, she has a noble heart, placing everyone else's needs above her own.