As a child he witnessed Jael "seducing" Kevan, which deeply traumatized him, and Jael's "accidental" death may have worsened the effect. As a result, he's a violent troublemaker, but his mother believes he can get better over time (and with the right marriage). Elodie can be forced into marrying him if faced with a civil war: Either she surrenders regency to Arisse and marries Thaddeus (causing a game over, Thaddeus's marriage epilogue is not shown), or she negotiates a compromise, elevating Arisse to Royal Advisor and becoming engaged to Thaddeus.
His insecurities are easily displayed in his epilogues:
- If Elodie pacified Togami, Thaddeus will suspect they have an affair, but he won't act on his suspicions beyond ocasionally brooding.
- If Elodie rejects Banion, he'll spreads rumors of Elodie having affairs, and if she can't disprove them, Thaddeus will quickly divorce her. However, Arisse can reveal Banion as the origin of said rumors, in which case he becomes the laughingstock of nobility and Elodie and Thaddeus stay together.
- If Elodie accepted Banion's proposal of an affair, Thaddeus will fiercely deny any evidence of Elodie's infidelity to himself and everyone else, even when their child is born "suspiciously fair-haired".