Fionn mac Cumhail

Fionn was the unparalleled leader of the Knights of Fianna of Erin, an "almighty group of knights", and friend of Diarmuid Ua Duibhne.
When Fionn was to be betrothed to his third wife and daughter of the High King, Gráinne, they put on a large banquet to celebrate the occasion, feeling as if the engagement was more joyous than any other. Due to Gráinne begging Diarmuid to take her away from there, the two ran away together, and Fionn, filled with jealousy and resentment, mobilized all of his resources to hunt them down.

In Fate/Grand Order:
~!Fionn serves as a part of the Celtic army under Medb. He is partnered with Diarmuid Ua Duibhne, whom he constantly reminds of his affair.
Fionn's appearance is of when he was at the prime of his life as a knight, brimming with elegance and charm as if he was shining. It reflects the time of his more youthful years before he had lost seven years of his youth from a conflict with a fairy over his first wife.!~

In Fate/Zero:
~!Fionn's appearance in the Fate/Zero anime is much more regal with him as a king, presumably after his youth was stolen by the fairy.!~

In Fate/Apocrypha:
~!Fionn is briefly shown during a flashback to the Third Holy Grail War, summoned by Darnic Prestone Yggdmillennia.!~