Little is known of his personal past. All that is known about him is that he was once a self-sacrificing and martyred saint. When he resurrected, he had no intention of playing martyr for anyone, and developed a bloodlust and thirst for power. He can be ruthless and merciless, but is surprisingly considered to be a respected, fair and a good leader by his army, many of whom often defect from other angel armies (including King Janos's) to follow him, and they have become surprisingly loyal and dedicated. He has the type of personality where he gains everyone’s loyalty, even Gabriel’s. Towards the author, however, he is possessive, lustful, and controlling, but fortunately, not abusive.
In Issue 5 where he was introduced, Jeromos wore his hair long, but cut it in Issue 6.