What does the latter part of this mean?

Cecil has never been a standout character to me, whatever character arc he had in the original game ended once he became a Paladin

See a translation

The latter part is a bit of a run-on sentence. It is also missing an important conjunction that would clarify the meaning. It is more easily understandable this way:
"Cecil has never been a standout character to me; and once he became a Paladin, whatever character arc he had in the original game ended."

The speaker is implying that Cecil did not have much of a character arc (in the original game). The speaker then adds their opinion: that when Cecil became a Paladin, it nullified his character arc from the previous game. (The implication is that Cecil's previous character arc somehow conflicts with his new role as a Paladin.)