Dragonmage appeared as a member of the Legion of Superheroes. He was referenced to in the original version, but only as a victim of Mordru (and not then as a Legionairre). He appeared as a member in the 1.5 version (post Crisis) joining with the SW-6 team, and as a member of the Legion-247 version. He has not appeared as a member in the reboots since then.
Xao Jin was born in the China sector of Earth, and his sensitivity to the forces of magic was apparent almost from the time of his birth. When his powers seriously threatened his family's safety during the Magic Wars, he was apprenticed to Chu Hua, a full-fledged magician, under whom he studied until the Dominators took him and his teacher captive.
He escaped the Dominators' underground chambers, thanks to Danielle Foccart, who sensed their impending destruction, and freed him, along with April Dumaka (Catspaw) and Bobb Kohan (Crystal Kid). His spells, which were always accompanied, for some reason, by dragon-shaped holograms, proved very useful while the four were on the run. He is not completely trained in the use of his power, and is easily rattled, an event that can lead to mishaps of large proportions. Nonetheless, he was inducted into the New Earth team of Legionnaires upon its formation, where he works on his concentration skills and is always willing to lend a hand. None of this helped, though, to prevent his disappearance at a pivotal point of a battle against Mordru and Glorith during Zero Hour.
However, he did not stay gone. Another version of his character appeared in the rebooted Legion book (Earth-247 version) as an applicant for Legion membership. The Legion turned him down for membership, explaining that sorcery was not an acceptable power for the Legion. This version of Dragonmage had a chip on his shoulder and went on a quest to prove his worth and to show up the Legion. He quickly got into trouble. He winds up as an apprentice to Mysa Nal, and together they help the Legion defeat Mordru.
This version is still quite headstrong and accidentally released some elemental forces which possessed some people (including Legionairre Sun Boy) that went on a rampage. Dragon Mage took responsibility for unleashing the forces and turned himself over to the authorities.
And then that version of the Legion got rebooted. Again. He has not been seen since then.
Power & Abilities
Dragonmage's schtick is sorcery. His early appearances lacked control. Later appearance demonstrated greater contol, but with bigger psyche disads. As noted above, all the uses of his power included the image of an Asian dragon.
After becoming a Legionairre he had access to a Legion Flight Ring and other Legion equipment. The second version was a Legion ally, but never became a Legionairre.