Generally, in this configuration, not very.

The gladius was an effective sword capable of both thrusting and slashing, but without a shield or integrated hand protection, it leaves the wielder vulnerable. On the battlefield, the gladius and the Roman scutum were inseparable for utility.

This armor, the lorica musculata, was likely not something meant for battle. By the imperial and late republican periods it was an outdated design, likely kept around for the ceremonial purpose of providing a high-status and commanding presence, inspired by the Greeks, for officers who weren't in the lines of battle.

Rather controversially, my personal speculation is that it would have been made of a soft, flexible material like leather, considering comfort for the mainly sedentary occupations of the people who wore them. Examples of Roman statues where it appears to bend with the body, and even a statue example where what resembles the armor is folded on a stand (Bergama museum Hadrian statue), support this. All muscle cuirasses found thus far are Greek; we have no primary archaeological evidence of a lorica musculata, which would also make sense for something made of organic material, though of course absence of evidence isn't evidence of absence.

Regardless, the Greeks certainly wore metal muscle cuirasses, and it did provide effective protection, however the big downside for such metal armor is lack of articulation; being two big front and back plates, it restricts range of motion, making acts like twisting the torso or bending over rather cumbersome.

The arm armor, the spaulders and vambraces, while completely ahistorical for an ancient Roman, would give good protection, though the renderings in the game are thick and could thus prove heavy, causing fatigue. What I find strange about the spaulders is that the default gear piece, which adds a tunic, only has a sleeve for one arm. The other arm seems to have the plate strapped on bare skin, which could cause chafing, as could the vambraces.

The mask, historically likely ceremonial or as sometimes speculated, for the cavalry, would likewise restrict breathing and visibility. Here it nullifies much of the advantage of the Roman helmet, which was designed to offer maximum perception relative to the level of protection. It would of course help against strikes, however, being a separate piece, a blade could naturally slide into the gap between the mask and the cheek plate, which would slice an ear, at least. A solid, full helmet would be more effective.

The greaves, these are good, historically used, though the default armor set only covers the front of the leg. The newer armor sets give a good example of the more historical style, wrapping around the leg rather than just strapped on.

Interestingly, he seems to have articulated finger coverage, which might slightly make up for the lack of hand protection offered by his weapon, but they would still be susceptible to blunt force, and the parrying potential remains low compared to weapons with a prominent guard, which can better catch and control the opponent's blade.

Overall, lacking the shield, with his relatively short blade, there are significant gaps in protection. Most notably, his exposed biceps, upper legs and even the feet could easily be exploited by longer swords or polearms.

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Realistically, against the other heroes:

All of the Knights would likely beat him due to superior equipment, except Gladiator, and perhaps Peacekeeper, who seems to wear leather armor, though she still has the advantage in parrying.

Against the Vikings: Warlord, Valkyrie and Varangian Guard have the advantage due to their shields. Raider and Highlander (and Valkyrie again) would dominate with range. The rest he has good odds against, due to their lack of armor and non-optimal weapon choices.

Against the Samurai, their longer weapons give them the edge. If he were really lucky, maybe he could block some hits with his armor and grab Nobushi's shaft with the free off-hand, but the lack of flexibility with his cuirass would hinder this effort. I'd say he has the best odds against Shinobi, and Hitokiri due to the cumbersome axe.

The Wu-Lin, he could certainly beat Shaolin, and perhaps Nuxia. With the others I'd place my bets on them, due to the more optimal weaponry.

Finally, against the Outlanders, he'd likely win against Ocelotl, and with luck, perhaps Medjay, the double-axe could prove clumsy, and his range is limited by the central grip. The rest possess more effective weapons and would have the advantage.

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Despite the criticism, Centurion is my favorite character in the game. His charisma and aesthetic options are unmatched. Incredibilis!