So, a Dungeons & Dragons paladin, then.
In the old days (1980s), there was very little attempt made to balance different fantasy character classes, and paladins were by far the most overpowered. This was on purpose, and the games’ designers made up for it by making the stat requirements to become a paladin so high that it was almost impossible to be one. (Remember, stats were randomly generated.) They were the superheroes of fantasy gaming, with crazy powers and a strict moral code.
Since this is an epic one-player game, maybe consider drawing from the old school, overpowered paladin: the one who is unstoppable with a holy magic sword, but who is forbidden from owning more wealth than they need.
Paladins also tended to creep people out because of their powers and their strict religious codes. A paladin could look into your soul and detect your evil intentions. Their passion for honor and virtue was great for leading armies, but not always good for making friends. When a paladin walked into a tavern, people tended to get real quiet.
It might be a good idea to have opposed stats for the paladin’s behavior (honor vs expedience, charity vs selfishness, strict vs lax, etc). An MC who is extremely virtuous might find some potential allies becoming alienated or distrustful, while an MC who is too lax might find their powers getting weaker. Traditionally, AD&D paladins who failed to live virtuously had their powers taken from them.
If this sounds like a good idea, you might want to google the 1st edition and 2nd edition AD&D paladin classes. The Neverwinter Nights and Baldur’s Gate paladins were watered down versions of this. You could even skim through the Complete Paladin’s Handbook, which has more ideas on paladins than anyone could possibly need.