This character uses an archaic dialect of their language. They use archaic words, grammar, phrases and so on. This could be for any number of reasons from being a time-traveler, having had a very strict, old-school upbringing or just doing it for style.

Examples of pronouns:
English: Thou, thee, thy, ...
Japanese: 吾 (ware), 我輩 (wagahai), 汝 (nanji).

Examples of verbs in English:
shalt, art, shouldst, etc.