Yuukaku similar to his wife, possess the typical "normal" Magus mindset, where one completely focuses and obsesses over obtaining and reaching the "Truth" by endlessly developing his and his wife's Magecraft while disregarding any form of restraint or morals in whatever method they deem "necessary" to accomplish this goal.
Yuukaku himself, alongside his wife, possessed little to no shame or hesitance in using his own firstborn daughter as a human guinea pig to further advanced his Magecraft and increase the magical potential of the future generations of his family by experimenting on her relentlessly. Eventually the end result of these tests left his daughter comatose, Yuukaku possess little to no guilt at his actions or empathy for his only child and was instead more focused on the fact whether or not her internal organs was still functional, and upon learning that her organs were undamaged, proceeded to happily declared to cut any form of treatment to cure her comatose state and instead agree to pay the needed fee to keep her hospitalized for the foreseeable future and instead planned to wait until she developed physically into the needed age in order to use her to produce a proper heir for their family's Magecraft and had no plans whatsoever to even try to help her recover from her coma.
However upon being forcibly dragged into the Tsubaki's Dream World by False Rider and infected by him, Yuukaku's personality was shown to have become completely changed and transformed into that of a loving and dotting father who loves and cares for his daughter very much. This change was shown to have completely overwritten his past personality, as seen when he was contacted by Faldeus Dioland, where he stated that he and his wife had no time to be messing around with the True and False Holy Grail Wars and instead should focus on caring for his daughter.
Although his exact abilities was never seen, Yuukaku alongside his wife, were both noted to be descended from powerful magical families and thus possessed immense magical pedigree's and aptitude for Magecraft. Having successfully obtained some of the mechanics for the Fuyuki Holy Grail and successfully stolen a piece of Zouken Matou's Entmomoancy Magecraft, speaks volumes of his knowledge and competence as a magus.
Eventually by further experimenting and adapting to the entomatic magecraft they obtained, he and his wife began making strides in creating a new method to alter flesh via using insects even smaller than the ones Zouken used. And after much trial and error, the couple successfully came up with pseudo-entomomancy in the form of magically-modified bacteria meant to be instilled within a young magus to amplify their Magic Circuits as they grow that successfully increased his daughter's overall Magic capacity and potential considerably, albeit at the cost of rendering her comatose.
Yuukaku was also shown to have been the intended Master for the False Holy Grail war instead of his daughter, thereby hinting that he was confident enough in his overall Magical Energy output to supply a Servant with Adequate magical energy for manifestation and for usage of its Noble Phantasm.
His abilities were said to have been high enough that when Faldeus Dioland noticed the rapid shift in his priorities and personality, he stated that only a magus with immense power and talent could have taken control of him.
Creation and Concept