Yuri is a Armor Knight one that graduated of a military academy that trains knights to protect Netherworld nobles. In addition to fighting, they are trained in etiquette. When they graduate, they are appointed to work for a noble and are given a set of obsidian armor, while very loyal and respectful for a demon she also wants to dominate him, be it as a toy or a pet either is fine but she is still very much so his loyal knight and protector putting her armored boot down on all that wish him harm