Selinuntius is Melos's steadfast friend, a stonemason and sculptor living in Syracuse. When Melos faces execution by King Dionysius, Selinuntius volunteers as a hostage, fully aware he will be executed if Melos fails to return within three days, demonstrating profound loyalty and trust despite the risk.

During imprisonment, he experiences significant internal conflict, grappling with doubt over Melos's return as the deadline approaches. This reveals his deep anxiety and the strength needed to maintain faith under pressure.

When Melos returns just in time, Selinuntius expresses remorse for his momentary loss of faith. He requests Melos strike him as atonement for doubting, simultaneously acknowledging his own weakness, highlighting his integrity and the mutual respect in their relationship.

Selinuntius's unwavering commitment, even facing potential betrayal, profoundly impacts King Dionysius. Witnessing their mutual trust and sacrifice challenges the king's cynical worldview, ultimately leading him to spare both men.

Expanded adaptations, like the 1992 film, explore additional layers including his professional struggles as a sculptor and personal challenges such as self-doubt and reliance on alcohol. These elements provide further context for his motivations and vulnerabilities, though his core role embodying sacrificial friendship remains consistent.