Zelos Wilder wields political power as Tethe'alla's Chosen, second only to the king. His childhood was defined by severe neglect after the Oracle of Cruxis forced his mother, Mylene Wilder, into marriage with his father, the prior Chosen, who maintained another relationship. The family butler, Sebastian, became his primary caregiver amidst this emotional isolation. Zelos formed a close early bond with his half-sister, Seles Wilder, but their relationship shattered after their father died. Seles's half-elven mother then attempted to assassinate Zelos to secure the Chosen title for her daughter, resulting in Mylene's death. Mylene's final condemnation of Zelos's existence fueled his profound self-loathing and created a lasting aversion to snow, which reminded him of the event. Though innocent, Seles was imprisoned for life in an abbey because of her mother's crime.

Zelos adopted a flamboyant, narcissistic, and relentlessly womanizing persona as a defense against his pain and societal pressures. This behavior helped him discern genuine individuals from those offering superficial adoration due to his status. At the Imperial Research Academy in Sybak, he manipulated peers to compensate for academic weaknesses, though he showed aptitude in mathematics. His status and appearance secured his graduation at the top of his class. Publicly expressing prejudice against half-elves—influenced by society and his mother's half-elven murderer—he privately held more sympathetic views, using his influence to block anti-half-elf legislation and forming genuine bonds with half-elf companions Genis and Raine Sage after recognizing their shared marginalization.

His role as the Chosen deepened his resentment toward the system, leading him to secretly ally with both Cruxis and the Renegades as a triple agent. He initially assisted Lloyd Irving's group under political pretexts while engineering traps like the Fooji Mountains ambush to further his hidden agendas. His betrayal crystallized in Flanoir; if Lloyd rejected others for a solitary walk, Zelos remained loyal, otherwise he defected to Cruxis. He later betrayed the group at the Tower of Salvation by handing Colette to Cruxis, though he subsequently rescued party members presumed dead. This revealed his deeper loyalty to Lloyd and exposed his triple-agent role to dismantle corrupt powers like the Pope's regime.

Zelos displayed perceptiveness beyond his frivolous facade, recognizing Colette's similar masking of emotional distress. He offered critical emotional support to Sheena Fujibayashi after Corrine's sacrificial death, intentionally using humor to divert the group's sorrow and prevent her despair. His strategic acumen surfaced when deceiving Papal Knights by exploiting their superstitions about Colette and persuading Princess Hilda to gain access to the half-elf village of Heimdall. He harbored distrust toward Kratos Aurion, possibly seeing parallels between Kratos's manipulation of Lloyd and his own father's abandonment.

After Mithos Yggdrasill's defeat, Zelos facilitated peace between the fused worlds of Sylvarant and Tethe'alla by appointing Sheena as an emissary. Two years later, in *Dawn of the New World*, he rescued Emil Castagnier and Marta Lualdi from Richter Abend, intervened to spare Alice after her attack, and identified an imposter Lloyd by detecting artificial scent differences. He rejoined the party temporarily to rescue the kidnapped Seles, further aiding in thwarting Richter's plan to open a gate to the monster world. Throughout these events, he maintained protective instincts toward Sheena, notably saving her from a collapsing interdimensional gate.