In P3, default Personae evidently are expected to follow a Classical Mythology scheme. Like Orpheus, Theseus went down into the Underworld on at least one occasion. But unlike Orpheus, who went in pursuit of his true love, Theseus went to the land of the dead to pick up chicks. Well, one chick in particular. It didn't work out all that well for him. Similarly, Shiki is often thought in supplementary materials to be a harem seeker, intentionally working to keep his cute lady friends around instead of decisively settling down with just one or two.
And given his track record with women - may or may not have dropped his first girlfriend at an island after his greatest victory; kidnapped Helen of Troy when the latter was a young girl, in an aborted attempt at wife husbandry; attempted to kidnap Persephone AFTER she was wedded to Hades; drove his wife to suicide due to trusting his son over her - I think it's fair to assume could Nasuverse!Theseus also have been a Lord of Bedroom Jerkassery in his own right.
Finally, Theseus is a famous ancient hero largely renowned for going around and killing people - first, assorted bandit assholes, and then the Minotaur.
And that's why I went with Theseus as Shiki's default Persona.
It is a physical-oriented Persona. Its current skills are Bash, and Sukukaja.
It looks like an Ancient Greek warrior, though it doesn't have much armor.