Going off portrait alone it's either a young emperor or, more likely, an empress. If it really is Roman my guess is empress Helena, mother of Constantine the Great. You could try matching the reverse to examples
here.
Notice I said if it really is Roman as I'm not sure, it could also be from other parts of the ancient world. Size and weight would also help here if nothing else at least determine the time period in which this could have been issued as Roman coins changed during the centuries. Try looking for some letters under a magnifying glass, even a few could help with identification.
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