I think the answer you are looking for is the Emperor Commodus. Many Roman emperors liked to think of themselves as gladiators and many of them participated to some extent in gladiatorial combat. But most of them did not really do so in any way that was likely to get them hurt and only did so once or twice.
By contrast, Commodus was much more into it. He didn't really risk much either, but he did a lot more gladiatorial stuff. He participated so much that he even made up a persona for himself. As a gladiator, he called himself Hercules.
Gladiators were trained fighter who fought one to one fight to death with each others in a form of sport for entertainment of Roman people. Most gladiators were prisoners of war, slaves and criminals who were forced to fight these contests. However some freemen also fought these contests for name and fame. One roman emperor, Commodus Antonius (161-192 A.D.) also fought as a Gladiator. However, his battles in the arena were reputed to be all fixed ones, where by prior arrangements the opponents agreed to loose the battle, whose lives were then spared by Commodus. Also he fought and slayed in the battle opponents who were already wounded prior to the battle.
Many emperors wanted to be gladiators, primarily because gladiators were adored by the common people and were famously known, whereas most Roman citizens at the time, had no idea whom the emperor was (in name only). Of the emperors who actually fought as gladiators, Caligula, Titus, Hadrian, Lucius Verus, Caracalla, Geta and Didius Julianus and Commodus, Commodus had fought the longest, over a five-year period, and one of the few emperor gladiators who fought animals as well as people. Herodotus describes Commodus as fighting ostriches, a hippopotamus and chained lions. Commodus was eventually strangled by one of his own servants.( http://www.enotes.com/topic/Commodus_(Gladiator)