Yūki Soleil is the twelve-year-old princess and national symbol of the distant Kingdom of Soleil. Orphaned in infancy by a terrorist attack that claimed both parents, she was raised in isolation within the royal palace. Trusted retainers, including educator Leila Etoile—who lost her own family in the same attack and became a maternal figure—oversaw her upbringing. Sheltered from her kingdom's harsh realities, Yūki believed her duty was solely to maintain happiness, her days filled with playful moments alongside childhood friend and guardian Joshua Ingram, punctuated by Leila's strict etiquette lessons. Political advisors Izana Langford and Harold Miller managed governance and shielded her completely from the ongoing war with the Grandiga Empire.

Yūki's sheltered existence shatters in the series' second episode when Joshua dies unexpectedly in combat; witnessing his corpse delivers her first brutal exposure to war and triggers profound grief. Subsequent events further erode her idealism: a naive attempt to rescue civilians leads directly to catastrophic military losses, starkly illustrating the consequences of her inexperience. By the midpoint, Yūki actively engages in strategy, controversially advocating surrender to prevent further bloodshed against her advisors' wishes. This period is marked by court betrayals and the successive deaths of most original retainers, including Harold and Izana. Her development culminates in a climactic decision to disable the advanced Chrars energy system sustaining her world's technology. This act halts the war but reverts society to pre-industrial capabilities, breeding societal resentment despite achieving peace.

The conclusion presents Yūki as a survivor bearing the emotional and physical scars of conflict, with only two close allies remaining. She shares a final moment of laughter with former enemy Stella Shining, symbolizing a hard-won reconciliation.