Rich, fairly young naval captain on extended shore leave. As the second-oldest son in his family, tradition dictated that Theseus Jason should pursue a naval career -- much against his own wishes. He breezed through officer training and gained his commission in 1912, just in time for the outbreak of the great war.
It soon emerged that Theseus was a captain of limited competence but boundless luck. His loyal crew were able to compensate for certain inaptitudes, and he was awarded the Victoria Cross in 1917 for accidentally capturing a German U-boat after his own vessel sank on top of it.
In the years after the war, Theseus finds himself at something of a loose end. His true calling is to be a bohemian playboy, but it's difficult for him to get into the right circles without trading on his naval connections, which he's reluctant to do, partly because he doesn't really like the navy and partly because it's a bit embarrassing when people find out he's a hopeless duffer at all that sailory stuff like swimming and tying knots.
Some unhealthy habits he picked up during the lonely nights in the north sea resurface from time to time, as he struggles to find his direction in life. Currently he has something of a drinking problem, as well as an addiction to newspaper puzzles, such as 'cross-words'.
Lives in London (family in Greenwich, of course), so he's most likely to be connected with Barleyhew and friends. They may well go to the same club. Perhaps Theseus is being wooed by some political types, because his illustrious naval career shows he's loyal and because being the club chess champion shows he's bright. Luckily they haven't yet seen him after a few bottles of Margaux. Theseus is happy to play along because it's a bit of a lark.