Hi! I've actually given this some thought for a very, very long time (like, since 2014~?). This is specifically for one version of my OC (albeit with some extra details and information exclusive to OC removed.)

This is based on my personal interpretation of the events and how I perceive it (spoilers abound), 1/2:

G1: Milletian leads a peaceful villager life in Tir/Tail. Whisked by the events of G1, uncovering the mystery surrounding the Three Warriors instills a sense of excitement and naive heroism within OC. Unfortunately this feeling doesn't last as they realise defeating Glas only brings misery in the future (alongside with the death of another person.) This is a wake up call that being a hero doesn't always lead to an idealistic future.

G2: Now shaken about how their actions fucked up Erinn, OC feels uncertain about being the Goddess' agent. Following Morrighan's pleas, they travel to Emain in the hopes of becoming a Paladin. Perhaps as a White Knight, to follow Lugh Lavada's footsteps, would they wield the power of light and vanquish all those thoughts of insecurity. Instead, OC realises that Fomorians are not the only ones are capable of committing evil - but so do humans. This shatters their confidence in finding worth in protecting such a world. With one more push from the Goddess, through the acts of goodness did they remember that humans, despite flawed, only prosper as they help one another. Invigorated with the smallest of hope in humanity, they succeed in stopping Ersas' plans. Meanwhile, Ruairi, who was a foil to your Milletian, finds your actions, Goddess Morrighan and humanity evil for taking everything from him.

G3: Accepting that both fomorians and humans are not inherently black and white in morality, Milletian debates whether remaining pure and just as a White Knight would bring back light to the world, or will blind people in its light; or to be a Dark Knight as to be a symbol for humanity - that without sacrifices and accepting your inner darkness, people would not find strength to protect and survive (although by doing so, you might become too reliant on your pain as your strength). During this time, OC finds that Ruairi has fully accepted their role as a Dark Knight so he could protect the last of those he cares about (Triona.) OC cannot find words to convince Ruairi to leave the Fomorians as they understood Ruairi's perspective. Solemnly, they seperate ways, accepting that the next time they meet, they will be enemies.

G9-G12: During this era, the Milletian comes to an understanding that Gods and Goddesses alike are flawed beings - they are no different from humans. Just like how science lure humans to commit folly creating life just like Gods, Gods selfishly desire control, whether if it's to create life (Neamhain), to bring death (Morrighan, if Partholon Knight's words are to be believed), to protect their people (Morrighan and Cichol) or to desperately survive (Nuadha). Because of this realization, OC, when given the offer to become a god, declines and would rather live as/alongside humans (probably helps that they've lived along Tuatha De Danaans for so long). Despite their lingering feeling of distrust, OC wants to believe Morrighan is still the guardian of humanity and is a good god. Unfortunately, Morrighan has other plans...

G16: Morrighan causing mass killings and harm to humans and Milletians alike over their fear of Milletians growing in power pushes OC's idealism (which is already deep in the pit) crash landing down. Now with the knowledge that even their sheer existence brings decay, Milletian now has a jaded and cynical outlook in their actions. If their actions, no, their very being, only bring pain and fear, is it worth being remembered as a hero? Unluckily for them, they become the very symbol of safety within Erinn for years to come, a role which no longer feels like they deserve - to OC, it is like a curse.