I have a particular character in my story who I feel is great. He's a retired monster hunter who settled down with his wife for his later years. He's all-around friendly, respectful, empathetic, wise, and still extremely strong compared to the average man (albeit pathetic compared to younger fighters). Overall, the typical Obi-Wan, who works as the conscience for my anti-hero and the passion for my reluctant hero.

But what irritates me is that I can find no obvious flaw in his character. He has bad manners (while remaining respectful of people), is extremely nostalgic for his old life and is often overlooked or forgotten by other characters. While those three traits are nice quirks, I don't really feel them to be flaws. How do I continue developing his character so that it has flaws, making him more well-rounded and realistic?