(Well, he's more of a Major character than a "protagonist", but he's is important to both of the other two protagonists' scenarios)
One of the last surviving descendents of Robert de Sable, and loyal member of the Templar and the Vatican Church, along with his assistant,
Maria de Luca, who harbors hidden feelings for him. Half-Italian/Half-French. Intelligent, Caring, Faithful, and never stops smiling. His late mother was implied to be a descendent of Claude des Armoises, and her family were safeguarding the Banner of Jeanne d'Arc (which serves as a catalyst, along with their chosen vessel). As a Templar member, Daniel's main weapon is "The White Key", which takes a form of a silver rosary, that can transform into a rapier/longsword in combat upon activation from channel prana through this holy relic. However, he, along with Maria, also carry some firearms that can fire conceptual bullets with casings made of silver blessed by the Vatican Church members.
Just after Fuyuki's Mass Destruction, while he's currently a Priest-in-training under his father's supervision, Daniel instantly fell into a coma for a few days. While in coma, he experienced unusual dreams and hearing a mysterious voice explaining something about the upcoming Prophecy and provide instructions to Daniel on summoning the "Saint" for this War (Ruler). Once he finally woken up, he found his upper body covered with "leftover Command Seals from the previous War". His family believed that what's left of the Pure Grail chose him as one of this New War's Overseers, and believed that the Prophecy that their ancestors predicted/theorized will come that they prepare to assist Pure Grail's summon of their second Overseer. Now, he acts as the Supervisors of this new Grail War along with Ruler (Jeanne d'Arc).
Even though Daniel is an Administrator of this war, what he desire from the Grail is to heal his beloved comatose sister,
Sophia (who is suffering a terminal illness since birth and is currently Jeanne's vessel in this war). Sophia had a hard time in her childhood due to her illness (and sorta being shunned for being the indirect cause of their mother's death in childbirth), but Daniel (along with a few others such as their father and Maria) was the only one who cared and loved Sophia and never blamed her for that indirect cause. That's why Daniel tried his best to take care of her while being a Templar member, in order for her to be happy in her Life and not to feel burdened about the past. He was stricken with grief when Sophia's condition starts to worsen and has fallen to a coma, and that's why Daniel is so determined to save her with the Grail's miracle, even through Ruler's warning, he'll pay any price to save her, so she can live Happily again, even if it means "he will no longer be part of her Life".
Daniel represents Hope.